Thin film transistor with offset structure and electrodes in a symmetrical arrangement

This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0053666, filed on Jun. 8, 2010, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.

1. Field

Aspects of embodiments according to the present invention relate to a thin film transistor and, more particularly, to a high-voltage thin film transistor with an offset structure.

2. Description of the Related Art

A thin film transistor (TFT) is a special type of a field effect transistor (FET) in which a thin semiconductor film is formed on an insulating supporting substrate. Similar to the FET, the TFT has three terminals, i.e., a gate, a drain, and a source, and performs switching as a main function. By adjusting a voltage that is applied to the gate, the TFT performs switching so that current flowing between the source and the drain may be in an “on” or “off” state.

A conventional high-voltage TFT is a type of TFT to which a high voltage is applied. The conventional high-voltage TFT may employ an offset structure so that it can be more resistant to the high voltage. A conventional offset structure has an offset region, which is a high-resistance region between a source region and a drain region. In general, if a high voltage is applied between the source region and the drain region, then the high voltage is also delivered to the high-resistance offset region, thereby preventing a high electric field from being formed on a channel layer. Accordingly, the TFT may be maintained at a sufficiently low level of “off” current I off .

However, since the high-resistance offset region is intentionally employed in the conventional high-voltage TFT in order to reduce bias stress caused when a high voltage is applied, the amount of “on” current I on may eventually decrease.

In order to solve or lessen the severity of a problem of a conventional high-voltage thin film transistor (TFT) having an offset structure whereby the amount of “on” current I on decreases, one or more embodiments of the present invention provide a thin film transistor in which a low “off” current and a high “on” current I on can be maintained at a sufficiently low level and a sufficiently high level, respectively.

In an exemplary embodiment of the present invention, a thin film transistor (TFT) is disclosed. The TFT includes an active region. The active region is divided into a first active region and a second active region. The active region includes a gate electrode, an active layer, a gate insulating layer, and a source/drain electrode layer. The active layer includes a first active layer and a second active layer. The first active layer corresponds to the first active region. The second active layer corresponds to the second active region. The first active layer and the second active layer overlap with the gate electrode. The gate insulating layer is between the gate electrode and the active layer. The source/drain electrode layer includes a first source/drain electrode, a second source/drain electrode, a third source/drain electrode, and a fourth source/drain electrode. The first and second source/drain electrodes are electrically connected to the first active layer. The third and fourth source/drain electrodes are electrically connected to the second active layer. Two source/drain electrodes selected from among the first to fourth source/drain electrodes overlap partially with the gate electrode. Other two source/drain electrodes from among the first to fourth source/drain electrodes are offset from the gate electrode. The first to fourth source/drain electrodes and the gate electrode are in a symmetrical arrangement.

The first and third source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the first and third source/drain electrodes. The second and fourth source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the second and fourth source/drain electrodes.

The first source/drain electrode and the third source/drain electrode may be connected to each other. The second source/drain electrode and the fourth source/drain electrode may be connected to each other.

The TFT may include two TFTs. Corresponding source/drain and gate electrodes of the two TFTs may be connected in a symmetrical arrangement to function as a single TFT.

The active layer may include a material selected from the group including amorphous silicon, polycrystalline silicon, microcrystalline silicon, an oxide semiconductor, an organic semiconductor, and combinations thereof.

The active region may further include an ohmic contact layer between the active layer and the source/drain electrode layer.

The first active layer may include a first source/drain region, a second source/drain region, and a first channel region. The first source/drain region corresponds to the first source/drain electrode. The second source/drain region corresponds to the second source/drain electrode. The first channel region is between the first source/drain region and the second source/drain region. The second active layer may include a third source/drain region, a fourth source/drain region, and a second channel region. The third source/drain region corresponds to the third source/drain electrode. The fourth source/drain region corresponds to the fourth source/drain electrode. The second channel region is between the third source/drain region and the fourth source/drain region.

The first channel region may include a first offset region. The first offset region does not overlap with any of the gate electrode, the first source/drain electrode, and the second source/drain electrode. The second channel region may include a second offset region. The second offset region does not overlap with any of the gate electrode, the third source/drain electrode, and the fourth source/drain electrode.

The source/drain electrode layer in the second active region may be rotationally symmetrical to the source/drain electrode layer in the first active region.

One of the first source/drain electrode and the second source/drain electrode may overlap with the gate electrode. An other of the first source/drain electrode and the second source/drain electrode may be offset from the gate electrode.

The first active region and the second active region may be insulated from each other.

The gate electrode may include a first gate electrode and a second gate electrode. The first gate electrode and the second gate electrode are parallel with each other.

The first and third source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the first and third source/drain electrodes. The second and fourth source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the second and fourth source/drain electrodes.

The source/drain electrode layer in the second active region may be rotationally symmetrical to the source/drain electrode layer in the first active region.

One of the first to fourth source/drain electrodes may overlap partially with the first gate electrode. An other of the first to fourth source/drain electrodes may overlap partially with the second gate electrode.

A width of the first gate electrode that overlaps with the second active layer may be smaller than a width of the first gate electrode that overlaps with the first active region.

The active region may further include an offset electrode. The offset electrode overlaps with a region between the first gate electrode and the second gate electrode. The offset electrode is insulated from the active layer.

The offset electrode may include a first offset electrode and a second offset electrode. The first offset electrode overlaps with a region between the first and second source/drain electrodes. The first offset electrode is insulated from the first active layer. The second offset electrode overlaps with a region between the third and fourth source/drain electrodes. The second offset electrode is insulated from the second active layer.

The first and third source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the first and third source/drain electrodes. The second and fourth source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the second and fourth source/drain electrodes.

The offset electrode may be electrically connected to the first gate electrode and the second gate electrode.

The first active region and the second active region may be insulated from each other.

In another exemplary embodiment according to the present invention, a thin film transistor (TFT) is disclosed. The TFT includes an active region. The active region is divided into a first active region, a second active region, and a third active region. The active region includes a gate electrode, an active layer, a gate insulating layer, and a source/drain electrode layer. The active layer includes a first active layer, a second active layer, and a third active layer. The first active layer corresponds to the first active region. The second active layer corresponds to the second active region. The third active layer corresponds to the third active region. The first active layer, the second active layer, and the third active layer overlap with the gate electrode. The gate insulating layer is between the gate electrode and the active layer. The source/drain electrode layer includes a first source/drain electrode, a second source/drain electrode, a third source/drain electrode, a fourth source/drain electrode, a fifth source/drain electrode, and a sixth source/drain electrode. The first source/drain electrode and the second source/drain electrode are electrically connected to the first active layer. The third source/drain electrode and the fourth source/drain electrode are electrically connected to the second active layer. The fifth source/drain electrode and the sixth source/drain electrode are electrically connected to the third active layer. Two source/drain electrodes selected from among the first to fourth source/drain electrodes overlap partially with the gate electrode. Other two source/drain electrodes from among the first to fourth source/drain electrodes are offset from the gate electrode. The first to sixth source/drain electrodes and the gate electrode are in a symmetrical arrangement.

The first, third, and fifth source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the first, third, and fifth source/drain electrodes. The second, fourth, and sixth source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the second, fourth, and sixth source/drain electrodes.

The source/drain electrode layer in the third active region may be symmetrical to the source/drain electrode layer in the first active region.

The first source/drain electrode and the second source/drain electrode may overlap partially with the gate electrode. The third source/drain electrode and the fourth source/drain electrode may be offset from the gate electrode.

The first active layer may include a first source/drain region, a second source/drain region, and a first channel region. The first source/drain region corresponds to the first source/drain electrode. The second source/drain region corresponds to the second source/drain electrode. The first channel region is between the first source/drain region and the second source/drain region. The second active layer may include a third source/drain region, a fourth source/drain region, and a second channel region. The third source/drain region corresponds to the third source/drain electrode. The fourth source/drain region corresponds to the fourth source/drain electrode. The second channel region is between the third source/drain region and the fourth source/drain region. The third active layer may include a fifth source/drain region, a sixth source/drain region, and a third channel region. The fifth source/drain region corresponds to the fifth source/drain electrode. The sixth source/drain region corresponds to the sixth source/drain electrode. The third channel region is between the fifth source/drain region and the sixth source/drain region.

The second channel region may include an offset region. The offset region does not overlap with any of the gate electrode, the third source/drain electrode, and the fourth source/drain electrode.

The source/drain electrode layer in the first active region, the source/drain electrode layer in the second active region, and the source/drain electrode layer in the third active region may be symmetrical along an axis of symmetry.

A width of the gate electrode that overlaps with the second active layer may be smaller than a width of the gate electrode that overlaps with the first and third active regions.

The first to third active regions may be insulated from one another.

In yet another exemplary embodiment according to the present invention, a thin film transistor (TFT) is disclosed. The TFT includes an active region. The active region is divided into a first active region, a second active region, a third active region, and a fourth active region. The active region includes a gate electrode, an active layer, a gate insulating layer, and a source/drain electrode layer. The active layer includes a first active layer, a second active layer, a third active layer, and a fourth active layer. The first active layer corresponds to the first active region. The second active layer corresponds to the second active region. The third active layer corresponds to the third active region. The fourth active layer corresponds to the fourth active region. The first active layer, the second active layer, the third active layer, and the fourth active layer overlap with the gate electrode. The gate insulating layer is between the gate electrode and the active layer. The source/drain electrode layer includes a first source/drain electrode, a second source/drain electrode, a third source/drain electrode, a fourth source/drain electrode, a fifth source/drain electrode, a sixth source/drain electrode, a seventh source/drain electrode, and an eighth source/drain electrode. The first source/drain electrode and the second source/drain electrode are electrically connected to the first active layer. The third source/drain electrode and the fourth source/drain electrode are electrically connected to the second active layer. The fifth source/drain electrode and the sixth source/drain electrode are electrically connected to the third active layer. The seventh source/drain electrode and the eighth source/drain electrode are electrically connected to the fourth active layer. Two source/drain electrodes selected from among the third to sixth source/drain electrodes overlap partially with the gate electrode. Other two source/drain electrodes from among the third to sixth source/drain electrodes are offset from the gate electrode. The first to eighth source/drain electrodes and the gate electrode are in a symmetrical arrangement.

The first, third, fifth, and seventh source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the first, third, fifth, and seventh source/drain electrodes. The second, fourth, sixth, and eighth source/drain electrodes may be electrically connected to each other so that a same voltage is applied to the second, fourth, sixth, and eighth source/drain electrodes.

The first active layer may include a first source/drain region, a second source/drain region, and a first channel region. The first source/drain region corresponds to the first source/drain electrode. The second source/drain region corresponds to the second source/drain electrode. The first channel region is between the first source/drain region and the second source/drain region. The second active layer may include a third source/drain region, a fourth source/drain region, and a second channel region. The third source/drain region corresponds to the third source/drain electrode. The fourth source/drain region corresponds to the fourth source/drain electrode. The second channel region is between the third source/drain region and the fourth source/drain region. The third active layer may include a fifth source/drain region, a sixth source/drain region, and a third channel region. The fifth source/drain region corresponds to the fifth source/drain electrode. The sixth source/drain region corresponds to the sixth source/drain electrode. The third channel region is between the fifth source/drain region and the sixth source/drain region. The fourth active layer may include a seventh source/drain region, an eighth source/drain region, and a fourth channel region. The seventh source/drain region corresponds to the seventh source/drain electrode. The eighth source/drain region corresponds to the eighth source/drain electrode. The fourth channel region is between the seventh source/drain region and the eighth source/drain region.

The second channel region may include a first offset region. The first offset region does not overlap with any of the gate electrode, the third source/drain electrode, and the fourth source/drain electrode. The third channel region may include a second offset region. The second offset region does not overlap with any of the gate electrode, the fifth source/drain electrode, and the sixth source/drain electrode.

The source/drain electrode layer in the third active region may be rotationally symmetrical to the source/drain electrode layer in the second active region. The source/drain electrode layer in the fourth active region may be symmetrical to the source/drain electrode layer in the first active region.

The first source/drain electrode and the second source/drain electrode may overlap partially with the gate electrode.

One of the third source/drain electrode and the fourth source/drain electrode may overlap with the gate electrode. An other of the third source/drain electrode and the fourth source/drain electrode may be offset from the gate electrode.

A width of the gate electrode that overlaps with the second and third active regions may be smaller than a width of the gate electrode that overlaps with the first and fourth active regions.

The first to fourth active regions may be insulated from one another.

In still yet another exemplary embodiment according to the present invention, a thin film transistor (TFT) is disclosed. The TFT includes an active region. The active region includes a gate electrode, an active layer, a gate insulating layer, and a source/drain electrode layer. The gate electrode includes a first gate electrode and a second gate electrode. The first gate electrode and the second gate electrode are parallel with each other. The active layer overlaps with the first gate electrode and the second gate electrode. The gate insulating layer is between the gate electrode and the active layer. The source/drain electrode layer includes a first source/drain electrode and a second source/drain electrode. The first source/drain electrode and the second source/drain electrode are electrically connected to the active layer. The first source/drain electrode overlaps partially with the first gate electrode. The second source/drain electrode overlaps partially with the second gate electrode. The first and second source/drain electrodes and the gate electrode are in a symmetrical arrangement.

The active region may further include an offset electrode. The offset electrode overlaps with a region between the first gate electrode and the second gate electrode. The offset electrode is insulated from the active layer.

The offset electrode may be electrically connected to the first gate electrode and the second gate electrode.

The active layer may include a first source/drain region, a second source/drain region, and a channel region. The first source/drain region corresponds to the first source/drain electrode. The second source/drain region corresponds to the second source/drain electrode. The channel region is between the first source/drain region and the second source/drain region.

The channel region may include an offset region. The offset region does not overlap with any of the first gate electrode, the second gate electrode, the first source/drain electrode, and the second source/drain electrode.

The above and other features and aspects of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

FIG. 1A is a layout diagram illustrating an active region for a thin film transistor (TFT) with an offset structure according to an embodiment of the present invention;

FIG. 1B is a cross-sectional view taken along a line I-I of FIG. 1A ;

FIG. 1C is a cross-sectional view taken along a line II-II of FIG. 1A ;

FIG. 2A is a layout diagram illustrating an active region for a TFT with an offset structure according to another embodiment of the present invention;

FIG. 2B is a cross-sectional view taken along a line I-I of FIG. 2A ;

FIG. 2C is a cross-sectional view taken along a line II-II of FIG. 2A ;

FIG. 3A is a layout diagram illustrating an active region for a TFT with an offset structure according to another embodiment of the present invention;

FIG. 3B is a cross-sectional view taken along a line I-I of FIG. 3A ;

FIG. 3C is a cross-sectional view taken along a line II-II of FIG. 3A ;

FIG. 3D is a cross-sectional view taken along a line III-III of FIG. 3A ;

FIG. 4A is a layout diagram illustrating an active region for a TFT with an offset structure according to another embodiment of the present invention;

FIG. 4B is a cross-sectional view taken along a line I-I of FIG. 4A ;

FIG. 4C is a cross-sectional view taken along a line II-II of FIG. 4A ;

FIG. 5A is a layout diagram illustrating an active region for a TFT with an offset structure according to another embodiment of the present invention;

FIG. 5B is a cross-sectional view taken along a line I-I of FIG. 5A ;

FIG. 5C is a cross-sectional view taken along a line II-II of FIG. 5A ;

FIG. 6A is a layout diagram illustrating an active region for a TFT with an offset structure according to another embodiment of the present invention;

FIG. 6B is a cross-sectional view taken along a line I-I of FIG. 6A ;

FIG. 7A is a layout diagram illustrating an active region for a TFT with an offset structure according to another embodiment of the present invention;

FIG. 7B is a cross-sectional view taken along a line I-I of FIG. 7A ;

FIG. 8 is a layout diagram illustrating an active region for a TFT with an offset structure according to another embodiment of the present invention;

FIG. 9 is a layout diagram illustrating an active region for a TFT with an offset structure according to another embodiment of the present invention; and

FIG. 10 is a graph illustrating results of a simulation in which the characteristics of a channel current versus a gate voltage of a TFT according to an embodiment of the present invention are compared with those of a comparative example.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will more thoroughly and completely convey features and aspects of the invention to those of ordinary skill in the art. In the drawings, the thickness of layers and regions may be exaggerated for clarity. In addition, like reference numerals represent like elements throughout the drawings.

In this disclosure, a source electrode and a drain electrode are not differentiated from each other and are described simply as source/drain electrodes. This is because in many situations (for example, in a display device), any one of the source/drain electrodes may act as either a source electrode or a drain electrode according to a voltage applied to a corresponding thin film transistor (TFT).

In this disclosure, the term “offset region” denotes a channel region in which source/drain electrodes and a gate electrode are separated from one another in a horizontal direction (e.g., a direction parallel to an underlying substrate). For example, the offset region may be a channel region between the gate electrode and one of the source/drain electrodes in the horizontal direction. Thus, a resistance value of the offset region is not reduced directly by a gate voltage and is instead maintained at a higher level than that of the channel region excluding the offset region (For example, the channel region that coincides with the gate electrode, but not with a source/drain electrode).

In addition, the term “overlapping region” denotes a source/drain region in which a source/drain electrode and a gate electrode overlap with one another (e.g., coincide in a direction normal to an underlying substrate). Finally, a source/drain electrode will be said to “overlap partially” with a gate electrode when a corresponding active layer contains both an overlapping region (e.g., gate electrode and source/drain electrode coinciding) and a channel region (e.g., gate electrode, but no source/drain electrode coinciding).

FIG. 1A is a layout diagram illustrating an active region 100 for a TFT with an offset structure according to an embodiment of the present invention. FIG. 1B is a cross-sectional view taken along a line I-I of FIG. 1A . FIG. 1C is a cross-sectional view taken along a line II-II of FIG. 1A .

Referring to FIGS. 1A to 1C , the active region 100 for the TFT has a vertical stacked structure in which a gate electrode 121 is formed on a substrate 111 and a gate insulating layer 123 is formed on the gate electrode 121 and the substrate 111 . An active layer 125 is formed on the gate insulating layer 123 , and first and second source/drain electrodes 133 a and 133 b are formed on the active layer 125 . Ohmic contact layers 131 a and 131 b are formed between the active layer 125 and the first and second source/drain electrodes 133 a and 133 b , respectively.

The substrate 111 may be formed of, for example, glass, quartz, plastic, silicon, ceramic, or metal. The gate electrode 121 may be formed of, but is not limited to, a conductive material selected from the group including Au, Ag, Cu, Ni, Pt, Pd, Al, Mo, W, Ti, and an alloy thereof. The gate insulating layer 123 may be formed of an insulating layer, such as a silicon oxide layer or a silicon nitride layer. The active layer 125 may be formed of, for example, amorphous silicon, polycrystalline silicon, microcrystalline silicon, an oxide semiconductor, an organic semiconductor, or combinations thereof. The first and second source/drain electrodes 133 a and 133 b may also be formed of, but are not limited to, a conductive material selected from the group including Au, Ag, Cu, Ni, Pt, Pd, Al, Mo, W, Ti, and an alloy thereof. An ohmic contact between each of the first and second source/drain electrodes 133 a and 133 b and the active layer 125 is formed using the ohmic contact layers 131 a and 131 b . The ohmic contact layers 131 a and 131 b may be, for example, high-density doped amorphous silicon layers.

As illustrated in FIG. 1A , the active region 100 is divided into a first active region 100 A and a second active region 100 B that are rotationally symmetrical to each other about a center of the active region 100 (e.g., rotating the active region 100 for half a revolution (180°) about its center results in the same layout). In the first active region 100 A, the first source/drain electrode 133 a overlaps partially with the gate electrode 121 , and the second source/drain electrode 133 b is separated from the gate electrode 121 by a distance d 1 in the horizontal direction (e.g., parallel to the substrate 111 ), as illustrated in FIG. 1B .

In the first active region 100 A, the active layer 125 includes a first source/drain region 125 a under the first source/drain electrode 133 a , a second source/drain region 125 b under the second source/drain electrode 133 b , and a first channel region 125 ch _ 1 between the first and second source/drain regions. The first channel region 125 ch _ 1 includes an offset region (corresponding to that portion of the first channel region 125 ch _ 1 marked off by the distance d 1 in FIG. 1B , as well as to the area between the gate electrode 121 and the second source/drain electrode 133 b marked off by the distance d 1 in FIG. 1A ) between the gate electrode 121 and the second source/drain electrode 133 b.

As illustrated in FIG. 1C , in the second active region 100 B, a third source/drain electrode 133 c is separated from the gate electrode 121 by the distance d 1 , and a fourth source/drain electrode 133 d overlaps partially with the gate electrode 121 . In the second active region 100 B, the active layer 125 includes a third source/drain region 125 c under the third source/drain electrode 133 c , a fourth source/drain region 125 d under the fourth source/drain electrode 133 d , and a second channel region 125 ch _ 2 between the third and fourth source/drain regions 125 c and 125 d . The second channel region 125 ch _ 2 includes an offset region (corresponding to that portion of the second channel region 125 ch _ 2 marked off by the distance d 1 in FIG. 1C , as well as to the area between the third source/drain electrode 133 c and the gate electrode 121 marked off by the distance d 1 in FIG. 1A ) between the third source/drain electrode 133 c and the gate electrode 121 .

The layout of the first source/drain electrode 133 a and the second source/drain electrode 133 b in the first active region 100 A is rotationally symmetrical to that of the third source/drain electrode 133 c and the fourth source/drain electrode 133 d in the second active region 100 B. The same rotational symmetry is also exhibited when the gate electrode 121 is considered, which accounts for the similar size in the offset regions and the overlapping regions of the first and second active regions 100 A and 100 B, as described further below. Ohmic contact layers 131 c and 131 d are formed between the active layer 125 and the third and fourth source/drain electrodes 133 c and 133 d , respectively.

The first source/drain electrode 133 a in the first active region 100 A is electrically connected to the third source/drain electrode 133 c in the second active region 100 B. Thus, the same voltage is applied from the same source to the first source/drain electrode 133 a and the third source/drain electrode 133 c . Likewise, the second source/drain electrode 133 b in the first active region 100 A is electrically connected to the fourth source/drain electrode 133 d in the second active region 100 B. Thus, the same voltage is applied from the same source to the second source/drain electrode 133 b and the fourth source/drain electrode 133 d.

In the first active region 100 A, if a high voltage is applied to the second source/drain electrode 133 b , then current flows from the first source/drain region 125 a to the first channel region 125 ch _ 1 and then to the second source/drain region 125 b . In this case, the offset region of the first channel region 125 ch _ 1 prevents a high electric field from being applied to the first channel region 125 ch _ 1 . Accordingly, it may be possible to prevent the performance of the TFT from degrading, thereby improving the “off” current characteristics thereof. In addition, the first source/drain region 125 a overlaps with the gate electrode 121 , thus improving the “on” current characteristics thereof.

If a direction in which the high voltage is applied is changed (for example, the high voltage is applied to the first source/drain electrode 133 a in the first active region 100 A), then current flows from the second source/drain region 125 b to the first channel region 125 ch _ 1 and then to the first source/drain region 125 a . In this case, a relative position of the offset region with respect to the source/drain region 125 a to which the high voltage is applied is different than when the high voltage is applied to the second source/drain electrode 133 b , since the offset region is adjacent to the second source/drain region 125 b but not to the first source/drain region 125 a . Thus, the intensity of an electric field formed in the first channel region 125 ch _ 1 and the amounts of “on” current and “off” current are also different than when the high voltage is applied to the second source/drain electrode 133 b.

That is, if only the first active region 100 A is considered, the relative position of the offset region in the first channel region 125 ch _ 1 with respect to the source/drain electrodes to which the high voltages are applied is different. In other words, when the high voltage is applied to the first source/drain electrode 133 a , the relative position of the offset region in the first channel region 125 ch _ 1 is different than when the same voltage is applied to the second source/drain electrode 133 b . As a result, the intensity of an electric field applied to the first channel region 125 ch _ 1 when the high voltage is applied to the first source/drain electrode 133 a is different than when the same voltage is applied to the second source/drain electrode 133 b . Thus, the amount of current flowing through the first channel region 125 ch _ 1 is not the same when the same voltage is applied to the first and second source/drain electrodes 133 a and 133 b , respectively. That is, the amount of current varies according to a direction in which the same voltage is applied. This may lead to an incorrect operation in the device employing the TFT. For example, if such a TFT is part of a display device, an error in gradation expression may occur, thus causing a display error.

On the other hand, when the active region 100 that includes the first active region 100 A and the second active region 100 B is considered, the layout of the second active region 100 B is rotationally symmetrical to that of the first active region 100 A. Thus, if a high voltage is applied to the second source/drain electrode 133 b in the first active region 100 A, the amount of current flowing through the first active region 100 A is equal to (or substantially equal to) the amount of current flowing through the second active region 100 B when the high voltage is applied to the third source/drain electrode 133 c in the second active region 100 B. Likewise, the amount of current flowing through the first active region 100 A when the high voltage is applied to the first source/drain electrode 133 a in the first active region 100 A is equal to (or substantially equal to) the amount of current flowing through the second active region 100 B when the high voltage is applied to the fourth source/drain electrode 133 d in the second active region 100 B.

Thus, the total amount of current flowing through the active region 100 when the high voltage is applied to the second source/drain electrode 133 b in the first active region 100 A and the fourth source/drain electrode 133 d in the second active region 100 B is substantially the same as when the same voltage is applied to the first source/drain electrode 133 a in the first active region 100 A and the third source/drain electrode 133 c in the second active region 100 B. That is, if a direction of the same voltage is changed, a direction of current changes, but the total amount of current flowing through the active region 100 is substantially the same.

In other embodiments, the first active region 100 A and the second active region 100 B may be insulated from each other so that each of them may not be influenced by current flowing through the other. The first active region 100 A and the second active region 100 B may be insulated from each other, for example, by applying an insulating layer to the active layer 125 . The shapes of the first active region 100 A and the second active region 100 B may vary according to the shape of the insulating layer applied to the active layer 125 .

In another embodiment, in the active region 100 , the first source/drain electrode 133 a may be connected to the third source/drain electrode 133 c and the second source/drain electrode 133 b may be connected to the fourth source/drain electrode 133 d . In this case, it is also possible to maintain an offset region, an overlapping region, and symmetry (for example, rotational symmetry) between the first active region 100 A and the second active region 100 B. In yet another embodiment, the first source/drain electrode 133 a may be connected to the third source/drain electrode 133 c and the second source/drain electrode 133 b may be connected to the fourth source/drain electrode 133 d within the active region 100 while the first active region 100 A is insulated from the second active region 100 B.

FIG. 2A is a layout diagram illustrating an active region 200 for a TFT with an offset structure according to another embodiment of the present invention. FIG. 2B is a cross-sectional view taken along a line I-I of FIG. 2A . FIG. 2C is a cross-sectional view taken along a line II-II of FIG. 2A . A vertical stacked structure of the TFT according to the current embodiment is similar to that of the TFT described above with reference to FIGS. 1A to 1C and thus, will not be described again here. Likewise, with FIGS. 2A-2C and the following figures, for conciseness of description, elements whose reference numerals differ only in the leading digit and whose structure is substantially the same as those described in earlier embodiments may not have such descriptions repeated.

As illustrated in FIG. 2A , the active region 200 is divided into a first active region 200 A, a second active region 200 B, and a third active region 200 C. The layout of the first active region 200 A is substantially the same as that of the third active region 200 C. In addition, the layout of each of the first active region 200 A, the second active region 200 B, and the third active region 200 C is symmetrical along a vertical axis of symmetry (e.g., the line perpendicular to the line II-II and bisecting the gate electrode 221 in FIG. 2A ). This type of symmetry is also referred to as “reflection symmetry” or “reflectional symmetry.” The layout of each of the first active region 200 A, the second active region 200 B, and the third active region 200 C is also symmetrical along a horizontal axis of symmetry (e.g., the line II-II in FIG. 2A ). Further, the layout of each of the first active region 200 A, the second active region 200 B, and the third active region 200 C is rotationally symmetrical about a center of the active region 200 . The width of the portions of a gate electrode 221 that overlap with the second active region 200 B may be smaller than the width of the portions of the gate electrode 221 that overlap with the first and third active regions 200 A and 200 C (as illustrated with dashed lines in FIG. 2A ).

As illustrated in FIG. 2B , in the first active region 200 A, a first source/drain electrode 233 a and a second source/drain electrode 233 b are symmetrical with respect to the gate electrode 221 while overlapping partially with the gate electrode 221 . In the first active region 200 A, an active layer 225 includes a first source/drain region 225 a under the first source/drain electrode 233 a , a second source/drain region 225 b under the second source/drain electrode 233 b , and a first channel region 225 ch _ 1 between the first and second source/drain regions 225 a and 225 b.

As illustrated in FIG. 2C , in the second active region 200 B, each of a third source/drain electrode 233 c and a fourth source/drain electrode 233 d is symmetrical with respect to the gate electrode 221 , and is separated from the gate electrode 221 by a distance d 2 . In the second active region 200 B, the active layer 225 includes a third source/drain region 225 c under the third source/drain electrode 233 c , a fourth source/drain region 225 d under the fourth source/drain electrode 233 d , and a second channel region 225 ch _ 2 between the third and fourth source/drain regions 225 c and 225 d . The second channel region 225 ch _ 2 also includes an offset region between the third source/drain electrode 233 c and the gate electrode 221 , and an offset region between the gate electrode 221 and the fourth source/drain electrode 233 d (corresponding to respective portions of the second channel region 225 ch _ 2 marked off by the distance d 2 in FIG. 2C , as well as to the areas between the gate electrode 221 and the third and fourth source/drain electrodes 233 c and 233 d marked off by the distance d 2 in FIG. 2A ).

As illustrated in FIG. 2A , in the third active region 200 C, a fifth source/drain electrode 233 e and a sixth source/drain electrode 233 f are symmetrical with respect to the gate electrode 221 while overlapping partially with the gate electrode 221 . The layout of the fifth and sixth source/drain electrodes 233 e and 233 f in the third active region 200 C is substantially the same as that of the first and second source/drain electrodes 233 a and 233 b in the first active region 200 A. Thus, similar to the structure of the first active region 200 A shown in FIG. 2B , in the third active region 200 C, the active layer 225 includes a fifth source/drain region 225 e under the fifth source/drain electrode 233 e , a sixth source/drain region 225 f under the sixth source/drain electrode 233 f , and a third channel region 225 ch _ 3 between the fifth and sixth source/drain regions 225 e and 225 f.

The first source/drain electrode 233 a in the first active region 200 A, the third source/drain electrode 233 c in the second active region 200 B, and the fifth source/drain electrode 233 e in the third active region 200 C are electrically connected to one another. Thus, the same voltage is applied from the same source to the first, third, and fifth source/drain electrodes 233 a , 233 c , and 233 e . Likewise, the second source/drain electrode 233 b in the first active region 200 A, the fourth source/drain electrode 233 d in the second active region 200 B, and the sixth source/drain electrode 233 f in the third active region 200 C are electrically connected to one another. Thus, the same voltage is applied from the same source to the second, fourth, and sixth source/drain electrodes 233 b , 233 d , and 233 f.

In the first active region 200 A, the first source/drain electrode 233 a and the second source/drain electrode 233 b are symmetrical with respect to the gate electrode 221 . In the second active region 200 B, the third source/drain electrode 233 c and the fourth source/drain electrode 233 d are symmetrical with respect to the gate electrode 221 . In the third active region 200 C, the fifth source/drain electrode 233 e and the sixth source/drain electrode 233 f are symmetrical with respect to the gate electrode 221 . Accordingly, even if a voltage applied to the first, third and fifth electrodes and a voltage applied to the second, fourth and sixth electrodes are interchanged, the total amount of current flowing through the active region 200 may be maintained at a constant level in the opposite direction.

In this case, in the first active region 200 A and the third active region 200 C, the source/drain regions 225 a , 225 b , 225 e , and 225 f overlap with both ends of the gate electrode 221 , thereby increasing the amount of “on” current. In the second active region 200 B, offset regions (corresponding to the areas between the gate electrode 221 and the third and fourth source/drain electrodes 233 c and 233 d marked off by the distance d 2 in FIGS. 2A and 2C ) are present in the second channel region 225 ch _ 2 since the third and fourth source/drain electrodes 233 c and 233 d are each separated from the gate electrode 221 by the distance d 2 . Thus, a high electric field may be prevented from being applied to the second channel region 225 ch _ 2 when a voltage is applied to the second active region 200 B. Accordingly, when the active region 200 is considered, it may be possible to improve the “off” current characteristics by preventing the performance of the TFT from degrading.

The first active region 200 A, the second active region 200 B, and the third active region 200 C are symmetrical with respect to the gate electrode 221 . Thus, if a direction of the same voltage is changed, a direction of current changes, but the total amount of current flowing through the active region 200 is substantially the same. Accordingly, according to the layout of the first to third active regions 200 A to 200 C, the total amount of current flowing through the active region 200 in the TFT according to the current embodiment may be the same when the same voltage is applied regardless of a direction in which the voltage is applied, thereby improving the “off” current characteristics and the “on” current characteristics.

In other embodiments, the first to third active regions 200 A to 200 C may be insulated from one another so that each of them may not be influenced by current flowing through the others. The first to third active regions 200 A to 200 C may be insulated from one another, for example, by applying an insulating layer to the active layer 225 . The shapes of the first to third active regions 200 A to 200 C may vary according to the shape of the insulating layer applied to the active layer 225 .

In another embodiment, in the active region 200 , the first source/drain electrode 233 a , the third source/drain electrode 233 c , and the fifth source/drain electrode 233 e may be connected to one another, and the second source/drain electrode 233 b , the fourth source/drain electrode 233 d , and the sixth source/drain electrode 233 f may be connected to one another. In this case, it is also possible to maintain an offset region, an overlapping region, and symmetry (e.g., reflectional symmetry, rotational symmetry) among the first to third regions 200 A to 200 C, and the total amount of current flowing through the active region 200 may thus be maintained. In yet another embodiment, the first source/drain electrode 233 a , the third source/drain electrode 233 c , and the fifth source/drain electrode 233 e may be connected to one another, and the second source/drain electrode 233 b , the fourth source/drain electrode 233 d , and the sixth source/drain electrode 233 f may be connected to one another within the active region 200 while the first active region 200 A, the second active region 200 B, and the third active region 200 C are insulated from one another.

FIG. 3A is a layout diagram illustrating an active region 300 for a TFT with an offset structure according to another embodiment of the present invention. FIG. 3B is a cross-sectional view taken along a line I-I of FIG. 3A . FIG. 3C is a cross-sectional view taken along a line II-II of FIG. 3A . FIG. 3D is a cross-sectional view taken along a line III-III of FIG. 3A . A vertical stacked structure of the TFT according to the current embodiment is similar to that of the TFT described above with reference to FIGS. 1A to 1C and thus, will not be described again here.

As illustrated in FIG. 3A , the active region 300 is divided into a first active region 300 A, a second active region 300 B, a third active region 300 C, and a fourth active region 300 D. The first active region 300 A and the fourth active region 300 D are substantially the same in terms of their layout and are symmetrical along a vertical axis of symmetry (for example, the line perpendicular to the line I-I and bisecting the gate electrode 321 in FIG. 3A ), along a horizontal axis of symmetry (for example, the line separating the second active region 300 B from the third active region 300 C), and about the center of the active region 300 (that is, rotational symmetry). The layout of the second active region 300 B is rotationally symmetrical to that of the third active region 300 C. The width of the portions of a gate electrode 321 that overlap with the second and third active regions 300 B and 300 C may be smaller than the width of the portions of the gate electrode 321 that overlap with the first and fourth active regions 300 A and 300 D (as illustrated with dashed lines in FIG. 3A ).

As illustrated in FIG. 3B , in the first active region 300 A, a first source/drain electrode 333 a and a second source/drain electrode 333 b are symmetrical with respect to the gate electrode 321 while overlapping partially with the gate electrode 321 . In the first active region 300 A, an active layer 325 includes a first source/drain region 325 a under the first source/drain electrode 333 a , a second source/drain region 325 b under the second source/drain electrode 333 b , and a first channel region 325 ch _ 1 between the first and second source/drain regions 325 a and 325 b.

As illustrated in FIG. 3C , in the second active region 300 B, a third source/drain electrode 333 c overlaps partially with the gate electrode 321 , and a fourth source/drain electrode 333 d is separated from the gate electrode 321 by a distance d 3 . In the second active region 300 B, the active layer 325 includes a third source/drain region 325 c under the third source/drain electrode 333 c , a fourth source/drain region 325 d under the fourth source/drain electrode 333 d , and a second channel region 325 ch _ 2 between the third and fourth source/drain regions 325 c and 325 d . The second channel region 325 ch _ 2 includes an offset region (corresponding to that portion of the second channel region 325 ch _ 2 marked off by the distance d 3 in FIG. 3C , as well as to the area between the gate electrode 321 and the fourth source/drain electrode 333 d marked off by the distance d 3 in FIG. 3A ) between the gate electrode 321 and the fourth source/drain electrode 333 d.

As illustrated in FIG. 3D , in the third active region 300 C, a fifth source/drain electrode 333 e is separated from the gate electrode 321 by the distance d 3 , and a sixth source/drain electrode 333 f overlaps partially with the gate electrode 321 . In the third active region 300 C, the active layer 325 includes a fifth source/drain region 325 e under the fifth source/drain electrode 333 e , a sixth source/drain region 325 f under the sixth source/drain electrode 333 f , and a third channel region 325 ch _ 3 between the fifth and sixth source/drain regions 325 e and 325 f . The third channel region 325 ch _ 3 includes an offset region (corresponding to that portion of the third channel region 325 ch _ 3 marked off by the distance d 3 in FIG. 3D , as well as to the area between the gate electrode 321 and the fifth source/drain electrode 333 e marked off by the distance d 3 in FIG. 3A ) between the fifth source/drain electrode 333 e and the gate electrode 321 .

The layout of the third source/drain electrode 333 c and the fourth source/drain electrode 333 d in the second active region 300 B is rotationally symmetrical to that of the fifth source/drain electrode 333 e and the sixth source/drain electrode 333 f in the third active region 300 C.

As illustrated in FIG. 3A , in the fourth active region 300 D, a seventh source/drain electrode 333 g and an eighth source/drain electrode 333 h are symmetrical with respect to the gate electrode 321 while overlapping partially with the gate electrode 321 . The layout of the seventh source/drain electrode 333 g and the eighth source/drain electrode 333 h in the fourth active region 300 D is substantially the same as that of the first source/drain electrode 333 a and the second source/drain electrode 333 b in the first active region 300 A. Thus, similar to the structure of the first active region 300 A shown in FIG. 3B , in the fourth active region 300 D, the active layer 325 includes a seventh source/drain region 325 g under the seventh source/drain electrode 333 g , an eighth source/drain region 325 h under the eighth source/drain electrode 333 h , and a fourth channel region 325 ch _ 4 between the seventh and eighth source/drain regions 325 g and 325 h.

The first source/drain electrode 333 a in the first active region 300 A, the third source/drain electrode 333 c in the second active region 300 B, the fifth source/drain electrode 333 e in the third active region 300 C, and the seventh source/drain electrode 333 g in the fourth active region 300 D are electrically connected to one another. Thus, the same voltage is applied from the same source to the first, third, fifth, and seventh source/drain electrodes 333 a , 333 c , 333 e , and 333 g . Likewise, the second source/drain electrode 333 b in the first active region 300 A, the fourth source/drain electrode 333 d in the second active region 300 B, the sixth source/drain electrode 333 f in the third active region 300 C, and the eighth source/drain electrode 333 h in the fourth active region 300 D are electrically connected to one another. Thus, the same voltage is applied from the same source to the second, fourth, sixth, and eighth source/drain electrodes 333 b , 333 d , 333 f , and 333 h.

In this case, in the first active region 300 A and the fourth active region 300 D, the source/drain regions 325 a , 325 b , 325 g , and 325 h overlap with both ends of the gate electrode 321 , thereby greatly improving the “on” current characteristics. In the second active region 300 B, the fourth source/drain electrode 333 d is separated from the gate electrode 321 by the distance d 3 and thus, the second channel region 325 ch _ 2 has the offset region corresponding to the area between the gate electrode 321 and the fourth source/drain electrode 333 d marked off by the distance d 3 in FIGS. 3A and 3C . In the third active region 300 C, the fifth source/drain electrode 333 e is separated from the gate electrode 321 by the distance d 3 and thus, the third channel region 325 ch _ 3 has the offset region corresponding to the area between the fifth source/drain electrode 333 e and the gate electrode 321 marked off by the distance d 3 in FIGS. 3A and 3D . Thus, a high electric field may be prevented from being applied to the second channel region 325 ch _ 2 and the third channel region 325 ch _ 3 since each of the second active region 300 B and the third active region 300 C have an offset region. Accordingly, when the active region 300 is considered, it may be possible to prevent the performance of the TFT from degrading, thereby improving the “off” current characteristics.

As described above, the first active region 300 A and the fourth active region 300 D are substantially the same in their layout and are symmetrical along an axis of symmetry in the vertical direction (and along a horizontal axis of symmetry, as well as being rotationally symmetrical), and the layout of the second active region 300 B is rotationally symmetrical to that of the third active region 300 C. Thus, when a direction of the same voltage is changed, a direction of current changes, but the total amount of current flowing through the active region 300 is substantially the same. According to the layout of the first to fourth active region 300 A to 300 D, the total amount of current flowing through the active region 300 in the TFT according to the current embodiment may be the same when the same voltage is applied regardless of a direction in which the voltage is applied, thus improving the “off” current characteristics and the “on” current characteristics.

In another embodiment, the first to fourth active regions 300 A to 300 D may be insulated from one another so that each of them may not be influenced by current flowing through the others. Also, as described above, in another embodiment, the first source/drain electrode 333 a , the third source/drain electrode 333 c , the fifth source/drain electrode 333 e , and the seventh source/drain electrode 333 g may be connected to one another, and the second source/drain electrode 333 b , the fourth source/drain electrode 333 d , the sixth source/drain electrode 333 f , and the eighth source/drain electrode 333 h may be connected to one another within the active region 300 while the first to fourth active regions 300 A to 300 D are insulated from one another.

FIG. 4A is a layout diagram illustrating an active region 400 for a TFT with an offset structure according to another embodiment of the present invention. FIG. 4B is a cross-sectional view taken along a line I-I of FIG. 4A . FIG. 4C is a cross-sectional view taken along a line II-II of FIG. 4A . A vertical stacked structure of the TFT according to the current embodiment is similar to that of the TFT described above with reference to FIGS. 1A to 1C and thus, will not be described again here.

As illustrated in FIG. 4A , the active region 400 is divided into a first active region 400 A and a second active region 400 B that are rotationally symmetrical to each other. The TFT according to the current embodiment has a double gate structure, in which two gate electrodes, namely a first gate electrode 421 a and a second gate electrode 421 b , are included in parallel within one active region 400 . The width of the portions of the first gate electrode 421 a that overlap with the second active region 400 B may be smaller than the width of the portions of the first gate electrode 421 a that overlap with the first active region 400 A, and the width of the portions of the second gate electrode 421 b that overlap with the first active region 400 A may be smaller than the width of the portions of the second gate electrode 421 b that overlap with the second active region 400 b (as illustrated with dashed lines in FIG. 4A ).

As illustrated in FIG. 4B , in the first active region 400 A, a first source/drain electrode 433 a overlaps partially with the first gate electrode 421 a , and a second source/drain electrode 433 b is separated from the second gate electrode 421 b by a distance d 4 . Thus, an active layer 425 includes a first source/drain region 425 a under the first source/drain electrode 433 a , a second source/drain region 425 b under the second source/drain electrode 433 b , and a first channel region 425 ch _ 1 between the first and second source/drain regions 425 a and 425 b . The first channel region 425 ch _ 1 includes an offset region (corresponding to the area between the first gate electrode 421 a and the second gate electrode 421 b along the line I-I in FIG. 4A ) between the first gate electrode 421 a and the second gate electrode 421 b , and an offset region (corresponding to that portion of the first channel region 425 ch _ 1 marked off by the distance d 4 in FIG. 4B , as well as to the area between the second gate electrode 421 b and the second source/drain electrode 433 b marked off by the distance d 4 in FIG. 4A ) between the second gate electrode 421 b and the second source/drain electrode 433 b.

As illustrated in FIG. 4C , in the second active region 400 B, a third source/drain electrode 433 c is separated from the first gate electrode 421 a by the distance d 4 , and a fourth source/drain electrode 433 d overlaps partially with the second gate electrode 421 b . Thus, the active layer 425 includes a third source/drain region 425 c under the third source/drain electrode 433 c , a fourth source/drain region 425 d under the fourth source/drain electrode 433 d , and a second channel region 425 ch _ 2 between the third and fourth source/drain regions 425 c and 425 d . The second channel region 425 ch _ 2 includes an offset region (corresponding to that portion of the second channel region 425 ch _ 2 marked off by the distance d 4 in FIG. 4C , as well as to the area between the third source/drain electrode 433 c and the first gate electrode 421 a marked off by the distance d 4 in FIG. 4A ) between the third source/drain electrode 433 c and the first gate electrode 421 a , and an offset region (corresponding to the area between the first gate electrode 421 a and the second gate electrode 421 b along the line II-II in FIG. 4A ) between the first gate electrode 421 a and the second gate electrode 421 b.

The layout of the first source/drain electrode 433 a and the second source/drain electrode 433 b in the first active region 400 A is rotationally symmetrical to that of the third source/drain electrode 433 c and the fourth source/drain electrode 433 d in the second active region 400 B. The same rotational symmetry is also exhibited when the first gate electrode 421 a and the second gate electrode 421 b are considered, which accounts for the similar size in the offset regions and overlapping regions in the first action region 400 A and the second active region 400 B. In the active region 400 , the first gate electrode 421 a and the second gate electrode 421 b are rotationally symmetrical to each other.

The first source/drain electrode 433 a in the first active region 400 A and the third source/drain electrode 433 c in the second active region 400 B are electrically connected to each other. Thus, the same voltage is applied from the same source to the first and third source/drain electrodes 433 a and 433 c . Likewise, the second source/drain electrode 433 b in the first active region 400 A and the fourth source/drain electrode 433 d in the second active region 400 B are electrically connected to each other. Thus, the same voltage is applied from the same source to the second and fourth source/drain electrodes 433 b and 433 d.

In the first active region 400 A, if a high voltage is applied to the second source/drain electrode 433 b , current flows from the first source/drain region 425 a to the first channel region 425 ch _ 1 and then to the second source/drain region 425 b . In this case, the first and second gate electrodes 421 a and 421 b reduce the amount of “off” current, and the offset regions in the first channel region 425 ch _ 1 prevent a high electric field from being applied to the first channel region 425 ch _ 1 . Thus, the performance of the TFT may be prevented from degrading, thereby improving the “off” current characteristics. In addition, the first source/drain region 425 a overlaps with the first gate electrode 421 a , thereby improving the “on” current characteristics.

If a direction in which the high voltage is applied is changed (for example, the high voltage is applied to the first source/drain electrode 433 a in the first active region 400 A), then current flows from the second source/drain region 425 b to the first channel region 425 ch _ 1 and then to the first source/drain region 425 a . In this case, relative positions of the offset regions with respect to the first source/drain region 425 a to which the high voltage is applied are different than when the high voltage is applied to the second source/drain electrode 433 b . Thus, the intensity of an electric field formed in the first channel region 425 ch _ 1 and the amounts of “on” current and “off” current are also different than when the high voltage is applied to the second source/drain electrode 433 b.

That is, if only the first active region 400 A is considered, the relative positions of the offset regions in the first channel region 425 ch _ 1 with respect to the source/drain electrodes to which the high voltages are applied is different, when the voltage applied to the first source/drain electrode 433 a and the voltage applied to the second source/drain electrode 433 b are interchanged. As a result, the intensity of an electric field applied to the first channel region 425 ch _ 1 is different when the high voltage is applied to the first source/drain electrode 433 a than when the same voltage is applied to the second source/drain electrode 433 b . Thus, the amount of current flowing through the first channel region 425 ch _ 1 is not the same when the same voltage is applied to the first and second source/drain electrodes 433 a and 433 b , respectively. That is, the amount of current varies according to a direction in which the same voltage is applied. In addition, due to the rotational symmetry in the layouts of the first and second active regions 400 A and 400 B, the same phenomenon also takes place when only the second active region 400 B is considered.

On the other hand, when the active region 400 including the first active region 400 A and the second active region 400 B is considered, the layout of the second active region 400 B is rotationally symmetrical to that of the first active region 400 A. Thus, if a high voltage is applied to the second source/drain electrode 433 b in the first active region 400 A, then the amount of current flowing through the first active region 400 A is equal to (or substantially equal to) the amount of current flowing through the second active region 400 B when the same high voltage is applied to the third source/drain electrode 433 c in the second active region 400 B. Likewise, if a high voltage is applied to the first source/drain electrode 433 a in the first active region 400 A, then the amount of current flowing through the first active region 400 A is equal to (or substantially equal to) the amount of current flowing through the second active region 400 B when the same high voltage is applied to the fourth source/drain electrode 433 d in the second active region 400 B.

Thus, the total amount of current flowing through the first and second active regions 400 A and 400 B when a high voltage is applied to the second source/drain electrode 433 b in the first active region 400 A and the fourth source/drain electrode 433 d in the second active region 400 B is substantially the same as when the same high voltage is applied to the first source/drain electrode 433 a in the first active region 400 A and the third source/drain electrode 433 c in the second active region 400 B. Thus, if a direction of the same voltage is changed, a direction of current changes, but the total amount of current flowing through the active region 400 remains substantially the same.

In other embodiments, the first and second active regions 400 A and 400 B may be insulated from each other so that each of them may not be influenced by current flowing through the other. In another embodiment, the first source/drain electrode 433 a may be connected to the third source/drain electrode 433 c and the second source/drain electrode 433 b may be connected to the fourth source/drain electrode 433 d within the active region 400 while the first and second active regions 400 A and 400 B are insulated from each other.

FIG. 5A is a layout diagram illustrating an active region 500 for a TFT with an offset structure according to another embodiment of the present invention. FIG. 5B is a cross-sectional view taken along a line I-I of FIG. 5A . FIG. 5C is a cross-sectional view taken along a line II-II of FIG. 5A . A vertical stacked structure of the TFT according to the current embodiment is similar to that of the TFT described above with reference to FIGS. 1A to 1C and thus, will not be described again here. The TFT according to the current embodiment is similar to the TFT of FIGS. 4A to 4C except that an offset electrode is formed between two gate electrodes, namely a first gate electrode 521 a and a second gate electrode 521 b . In some embodiments, like the one shown in FIG. 5A , the offset electrode includes two offset electrodes.

As illustrated in FIG. 5A , the active region 500 is divided into a first active region 500 A and a second active region 500 B that are rotationally symmetrical to each other. The TFT according to the current embodiment has a double gate structure, in which the first and second gate electrodes 521 a and 521 b are included in parallel within one active region 500 . The width of the portions of the first gate electrode 521 a that overlap with the second active region 500 B may be smaller than the width of the portions of the first gate electrode 521 a that overlap with the first active region 500 A, and the width of the portions of the second gate electrode 521 b that overlap with the first active region 500 A may be smaller than the width of the portions of the second gate electrode 521 b that overlap with the second active region 500 b (as illustrated with dashed lines in FIG. 5A ).

As illustrated in FIG. 5B , in the first active region 500 A, a first source/drain electrode 533 a overlaps partially with the first gate electrode 521 a , and a second source/drain electrode 533 b is separated from the second gate electrode 521 b by a distance d 5 . Thus, an active layer 525 includes a first source/drain region 525 a under the first source/drain electrode 533 a , a second source/drain region 525 b under the second source/drain electrode 533 b , and a first channel region 525 ch _ 1 between the first and second source/drain regions 525 a and 525 b . The first channel region 525 ch _ 1 includes an offset region (corresponding to the area between the first gate electrode 521 a and the second gate electrode 521 b along the line I-I in FIG. 5A , including the area corresponding to a first offset electrode 533 o _ 1 ) between the first gate electrode 521 a and the second gate electrode 521 b , and an offset region (corresponding to that portion of the first channel region 525 ch _ 1 marked off by the distance d 5 in FIG. 5B , as well as to the area between the second gate electrode 521 b and the second source/drain electrode 533 b marked off by the distance d 5 in FIG. 5A ) between the second gate electrode 521 b and the second source/drain electrode 533 b.

The first offset electrode 533 o _ 1 is located between the first gate electrode 521 a and the second gate electrode 521 b to be insulated from the active layer 525 . The first offset electrode 533 o _ 1 may be electrically connected to the first and second gate electrodes 521 a and 521 b . A first offset insulation layer 531 o _ 1 is formed between the first channel region 525 ch _ 1 and the first offset electrode 533 o _ 1 to insulate the first offset electrode 533 o _ 1 from the active layer 525 .

As illustrated in FIG. 5C , in the second active region 500 B, a third source/drain electrode 533 c is separated from the first gate electrode 521 a by the distance d 5 , and a fourth source/drain electrode 533 d overlaps partially with the second gate electrode 521 b . In the second active region 500 B, the active layer 525 includes a third source/drain region 525 c under the third source/drain electrode 533 a , a fourth source/drain region 525 d under the fourth source/drain electrode 533 b , and a second channel region 525 ch _ 2 between the third and fourth source/drain regions 525 c and 525 d . The second channel region 525 ch _ 2 includes an offset region (corresponding to that portion of the second channel region 525 ch _ 2 marked off by the distance d 5 in FIG. 5C , as well as to the area between the third source/drain electrode 533 c and the first gate electrode 521 a marked off by the distance d 5 in FIG. 5A ) between the third source/drain electrode 533 c and the first gate electrode 521 a , and an offset region (corresponding to the area between the first gate electrode 521 a and the second gate electrode 521 b along the line II-II in FIG. 5A , including the area corresponding to a second offset electrode 533 o _ 2 ) between the first gate electrode 521 a and the second gate electrode 521 b.

The second offset electrode 533 o _ 2 is located between the first gate electrode 521 a and the second gate electrode 521 b to be insulated from the active layer 525 . The second offset electrode 533 o _ 2 may be electrically connected to the first and second gate electrodes 521 a and 521 b . A second offset insulation layer 531 o _ 2 is formed between the second channel region 525 ch _ 2 and the second offset electrode 533 o _ 2 to insulate the second offset electrode 533 o _ 2 from the active layer 525 .

The layout of the first source/drain electrode 533 a and the second source/drain electrode 533 b in the first active region 500 A is rotationally symmetrical to that of the third source/drain electrode 533 c and the fourth source/drain electrode 533 d in the second active region 500 B. The same rotational symmetry is also exhibited when the first gate electrode 521 a and the second gate electrode 521 b are considered, which accounts for the similar size in the offset regions and overlapping regions in the first action region 500 A and the second active region 500 B. In the active region 500 , the first gate electrode 521 a and the second gate electrode 521 b are rotationally symmetrical to each other.

The first source/drain electrode 533 a in the first active region 500 A and the third source/drain electrode 533 c in the second active region 500 B are electrically connected to each other. Thus, the same voltage is applied from the same source to the first and third source/drain electrode 533 a and 533 c . Likewise, the second source/drain electrode 533 b in the first active region 500 A and the fourth source/drain electrode 533 d in the second active region 500 B are electrically connected to each other. Thus, the same voltage is applied from the same source to the second and fourth source/drain electrode 533 b and 533 d.

In the first active region 500 A, if a high voltage is applied to the second source/drain electrode 533 b , then current flows from the first source/drain region 525 a to the first channel region 525 ch _ 1 and then to the second source/drain region 525 b . In this case, the first and second gate electrodes 521 a and 521 b reduce the amount of “off” current, and the offset regions in the first channel region 525 ch _ 1 prevent a high electric field from being applied to the first channel region 525 ch _ 1 . In addition, the first offset electrode 533 o _ 1 may control a resistance value of the offset region between the first gate electrode 521 a and the second gate electrode 521 b in such a manner that current may smoothly flow through the offset region. Therefore, the performance of the TFT may be prevented from degrading, thus improving the “off” current characteristics. Further, the first source/drain region 525 a overlaps with the first gate electrode 521 a , thus improving the “on” current characteristics.

If a direction in which the high voltage is applied is changed (for example, the high voltage is applied to the first source/drain electrode 533 a in the first active region 500 A), then current flows from the second source/drain region 525 b to the first channel region 525 ch _ 1 and then to the first source/drain region 525 a . In this case, relative positions of the offset regions with respect to the first source/drain region 525 a to which the high voltage is applied are different than when the high voltage is applied to the second source/drain electrode 533 b . Thus, the intensity of an electric field formed in the first channel region 525 ch _ 1 and the amounts of “on” current and “off” current are also different than when the high voltage is applied to the second source/drain electrode 533 b.

That is, if only the first active region 500 A is considered, the relative positions of the offset regions of the first channel region 525 ch _ 1 with respect to the source/drain electrodes to which the high voltages are applied is different, when the voltage applied to the first source/drain electrode 533 a and 533 c and the voltage applied to the second source/drain electrode 533 b are interchanged. As a result, the intensity of an electric field applied to the first channel region 525 ch _ 1 is different when the high voltage is applied to the first source/drain electrode 533 a than when the same voltage is applied to the second source/drain electrode 533 b . Thus, the amount of current flowing through the first channel region 525 ch _ 1 is not the same when the same voltage is applied to the first and second source/drain electrodes 533 a and 533 b , respectively. That is, the amount of current varies according to a direction in which the same voltage is applied. In addition, due to the rotational symmetry in the layouts of the first and second active regions 500 A and 500 B, the same phenomenon also takes place when only the second active region 500 B is considered.

On the other hand, when the active region 500 including the first active region 500 A and the second active region 500 B is considered, the layout of the second active region 500 B is rotationally symmetrical to that of the first active region 500 A. Thus, if a high voltage is applied to the second source/drain electrode 533 b in the first active region 500 A, then the amount of current flowing through the first active region 500 A is equal to (or substantially equal to) the amount of current flowing through the second active region 500 B when the same high voltage is applied to the third source/drain electrode 533 c in the second active region 500 B. Likewise, if a high voltage is applied to the first source/drain electrode 533 a in the first active region 500 A, then the amount of current flowing through the first active region 500 A is equal to (or substantially equal to) the amount of current flowing through the second active region 500 B when the same high voltage is applied to the fourth source/drain electrode 533 d in the second active region 500 B.

Thus, the total amount of current flowing through the first and second active regions 500 A and 500 B when a high voltage is applied to the second source/drain electrode 533 b in the first active region 500 A and the fourth source/drain electrode 533 d in the second active region 500 B is substantially the same as when the same high voltage is applied to the first source/drain electrode 533 a in the first active region 500 A and the third source/drain electrode 533 c in the second active region 500 B. That is, if a direction of the same voltage is changed, a direction of current changes, but the total amount of current flowing through the active region 500 remains substantially the same.

In other embodiments, the first and second active regions 500 A and 500 B may be insulated from each other so that each of them may not be influenced by current flowing through the other. In another embodiment, the first source/drain electrode 533 a may be connected to the third source/drain electrode 533 c , the second source/drain electrode 533 b may be connected to the fourth source/drain electrode 533 d , and the first offset electrode 533 o _ 1 may be connected to the second offset electrode 533 o _ 2 within the active region 500 while the first and second active regions 500 A and 500 B are insulated from each other.

FIG. 6A is a layout diagram illustrating an active region 600 for a TFT with an offset structure according to another embodiment of the present invention. FIG. 6B is a cross-sectional view taken along a line I-I of FIG. 6A . A vertical stacked structure of the TFT according to the current embodiment is similar to that of the TFT described above with reference to FIGS. 1A to 1C and thus, will not be described again here. The TFT according to the current embodiment has a double gate structure, in which two gate electrodes, namely a first gate electrode 621 a and a second gate electrode 621 b , are included in parallel within one active region 600 .

As illustrated in FIG. 6B , in the active region 600 , a first source/drain electrode 633 a overlaps partially with the first gate electrode 621 a , and a second source/drain electrode 633 b overlaps partially with the second gate electrode 621 b . The first source/drain electrode 633 a is separated from the second gate electrode 621 b by a distance d 6 and thus, the first source/drain electrode 633 a may be considered as being offset from the second gate electrode 621 b . Likewise, the second source/drain electrode 633 b is separated from the first gate electrode 621 a by the distance d 6 and thus, the second source/drain electrode 633 b may be considered as being offset from the first gate electrode 621 a.

An active layer 625 includes a first source/drain region 625 a under the first source/drain electrode 633 a , a second source/drain region 625 b under the second source/drain electrode 633 b , and a channel region 625 ch between the first and second source/drain regions 625 a and 625 b . The channel region 625 ch includes an offset region (corresponding to the area between the first gate electrode 621 a and the second gate electrode 621 b along the line I-I in FIG. 6A , including the area corresponding to the intersection of the two regions marked off by the distance d 6 ) between the first gate electrode 621 a and the second gate electrode 621 b.

In the active region 600 , the layout of the first source/drain electrode 633 a is symmetrical to that of the second source/drain electrode 633 b along a vertical axis of symmetry (for example, the line perpendicular to the line I-I and midway between the first gate electrode 621 a and the second gate electrode 621 b in FIG. 6A ). In addition, the layout of the first gate electrode 621 a is also symmetrical to that of the second gate electrode 621 b along the same vertical axis of symmetry. The same symmetry for both the source/drain electrodes and the gate electrodes is also exhibited along a horizontal axis of symmetry (for example, the line I-I in FIG. 6A ). Further, these sets of electrodes are also rotationally symmetrical.

In the active region 600 , if a high voltage is applied to the second source/drain electrode 633 b , then current flows from the first source/drain region 625 a to the channel region 625 ch and then to the second source/drain region 625 b . In this case, the first and second gate electrodes 621 a and 621 b reduce the amount of “off” current, and the offset region of the channel region 625 ch prevents a high electric field from being applied to the channel region 625 ch . Therefore, the performance of the TFT may be prevented from degrading, thereby improving the “off” current characteristics. In addition, the first source/drain region 625 a overlaps with the first gate electrode 621 a , thus improving the “on” current characteristics. Similarly, the second source/drain region 625 b overlaps with the second gate electrode 621 b , thus improving the “on” current characteristics.

If a direction in which the high voltage is applied is changed (for example, the high voltage is applied to the first source/drain electrode 633 a in the first active region 600 ), then current flows from the second source/drain region 625 b to the channel region 625 ch and then to the first source/drain region 625 a . In this case, a relative position of the offset region with respect to the source/drain region 625 a to which the high voltage is applied is the same as when the high voltage is applied to the second source/drain electrode 633 b . Thus, current flowing through the active region 600 is different in terms of a direction thereof but is substantially the same in terms of the amount thereof, compared to when the high voltage is applied to the second source/drain electrode 633 b . That is, the amount of current is substantially the same regardless of the direction in which the voltage is applied when the magnitude of the voltage is the same for each direction.

FIG. 7A is a layout diagram illustrating an active region 700 for a TFT with an offset structure according to another embodiment of the present invention. FIG. 7B is a cross-sectional view taken along a line I-I of FIG. 7A . A vertical stacked structure of the TFT according to the current embodiment is similar to the TFT described above with reference to FIGS. 1A to 1C and thus, will not be described again here. The TFT according to the current embodiment has a double gate structure, in which two gate electrodes, namely a first gate electrode 721 a and a second gate electrode 721 b , are included in parallel within one active region 700 .

As illustrated in FIG. 7B , in the active region 700 , a first source/drain electrode 733 a overlaps partially with the first gate electrode 721 a , and a second source/drain electrode 733 b overlaps partially with the second gate electrode 721 b . The first source/drain electrode 733 a is separated from the second gate electrode 721 b by a distance d 7 and thus, the first source/drain electrode 733 a may be considered as being offset from the second gate electrode 721 b . Likewise, the second source/drain electrode 733 b is separated from the first gate electrode 721 a by the distance d 7 and thus, the second source/drain electrode 733 b may be considered as being offset from the first gate electrode 721 a.

An active layer 725 includes a first source/drain region 725 a under the first source/drain electrode 733 a , a second source/drain region 725 b under the second source/drain electrode 733 b , and a channel region 725 ch between the first and second source/drain regions 725 a and 725 b . The channel region 725 ch includes an offset region (corresponding to the area between the first gate electrode 721 a and the second gate electrode 721 b along the line I-I in FIG. 7A , including the area corresponding to an offset electrode 733 o ) between the first gate electrode 721 a and the second gate electrode 721 b.

The offset electrode 733 o is located between the first gate electrode 721 a and the second gate electrode 721 b to be insulated from the active layer 725 . The offset electrode 733 o may be electrically connected to the first and second gate electrodes 721 a and 721 b . An offset insulation layer 731 o is formed between the channel region 725 ch and the offset electrode 733 o to insulate the offset electrode 733 o from the active layer 725 .

In the active region 700 , the layout of the first source/drain electrode 733 a is symmetrical to that of the second source/drain electrode 733 b along a vertical axis of symmetry (for example, the line perpendicular to the line I-I and bisecting the offset electrode 733 o in FIG. 7A ). In addition, the layout of the first gate electrode 721 a is also symmetrical to that of the second gate electrode 721 b along the same vertical axis of symmetry. The same symmetry between these sets of electrodes is exhibited along a horizontal axis of symmetry (e.g., line I-I of FIG. 7A ), and there is also rotational symmetry between these sets of electrodes.

In the active region 700 , if a high voltage is applied to the second source/drain electrode 733 b , then current flows from the first source/drain region 725 a to the channel region 725 ch and then to the second source/drain region 725 b . In this case, the first and second gate electrodes 721 a and 721 b reduce the amount of “off” current, and the offset region in the channel region 725 ch prevents a high electric field from being applied to the channel region 725 ch . Accordingly, the performance of the TFT may be prevented from being degraded, thereby improving the “off” current characteristics. In addition, the offset electrode 733 o may control a resistance value of the offset region between the first gate electrode 721 a and the second gate electrode 721 b in such a manner that current may smoothly flow through the offset region. Further, the first source/drain region 725 a overlaps with the first gate electrode 721 a and the second source/drain region 725 b overlaps with the second gate electrode 721 b , thereby improving the “on” current characteristics.

If a direction in which the high voltage is changed (for example, the high voltage is applied to the first source/drain electrode 733 a in the active region 700 ), then current flows from the second source/drain region 725 b to the channel region 725 ch and then to the first source/drain region 725 a . In this case, a relative position of the offset region with respect to the source/drain region 725 a to which the high voltage is applied, is the same as when the high voltage is applied to the second source/drain electrode 733 b . Thus, current flowing through the active region 700 is different in terms of a direction thereof but is substantially the same in terms of the amount thereof, compared to when the high voltage is applied to the second source/drain electrode 733 b . That is, the amount of current is substantially the same regardless of the direction in which the voltage is applied when the magnitude of the voltage is the same for each direction.

FIG. 8 is a layout diagram illustrating an active region 800 for a TFT with an offset structure according to another embodiment of the present invention. The active region 800 is divided into a first active region 800 A and a second active region 800 B that are rotationally symmetrical to each other.

Referring to FIG. 8 , the active region 800 has the same structure as the active region 100 of FIG. 1A , except that a first source/drain electrode 833 a is a combination of the first source/drain electrode 133 a and the third source/drain electrode 133 c of the active region 100 illustrated in FIG. 1A , and a second source/drain electrode 833 b is a combination of the second source/drain electrode 133 b and the fourth source/drain electrode 133 d of the active region 100 . Thus, a cross-sectional view of the TFT of FIG. 8 taken along a line I-I of FIG. 8 is the same as the cross-sectional view of FIG. 1B , and a cross-sectional view of the TFT of FIG. 8 taken along a line II-II of FIG. 8 is the same as the cross-sectional view of FIG. 1C .

In the first active region 800 A, the first source/drain electrode 833 a overlaps partially with a gate electrode 821 and the second source/drain electrode 833 b is separated from the gate electrode 821 by a distance d 8 . In the second active region 800 B, the first source/drain electrode 833 a is separated from the gate electrode 821 by the distance d 8 , and the second source/drain electrode 833 b overlaps partially with the gate electrode 821 .

As described above with respect to the active region 100 of FIGS. 1A to 1C , in the TFT of FIG. 8 , a high electric field may be prevented from being applied to a channel region due to an offset region between the gate electrode 821 and one of the source/drain electrodes, each of which is separated from the gate electrode 821 by the distance d 8 . Thus, the performance of the TFT may be prevented from degrading, thus improving the “off” current characteristics. In addition, both the first source/drain electrode 833 a and the second source/drain electrode 833 b overlap partially with the gate electrode 821 , thereby improving the “on” current characteristics. Since the first source/drain electrode 833 a , the second source/drain electrode 833 b , and the gate electrode 821 are rotationally symmetrical about the center of the active region 800 , the amount of current flowing through a channel region is substantially the same regardless of a direction in which a voltage is applied to the first source/drain electrode 833 a and the second source/drain electrode 833 b , when the magnitude of the voltage is the same for each direction.

Although the first and second source/drain electrodes 833 a and 833 b are connected to each other in the first active region 800 A and the second active region 800 B, it is possible to limit a path of current by insulating an active layer 825 .

FIG. 9 is a layout diagram illustrating an active region 900 for a TFT with an offset structure according to another embodiment of the present invention. The active region 900 is divided into a first active region 900 A and a second active region 900 B that are symmetrical to each other along a horizontal axis of symmetry (for example, the line separating the first active region 900 A and the second active region 900 B).

The active region 900 is a combination of two active regions 800 of FIG. 8 , with the layout of one active region 800 being the mirror image (reflection) of the layout of the other active region 800 . Specifically, the active region 900 has a structure in which the two active regions 800 of FIG. 8 are separated from each other by a distance (for example, a predetermined distance) and symmetrical to each other along the horizontal axis of symmetry (for example, the line separating the first active region 900 A and the second active region 900 B). In addition, bottom and top surfaces of the first and second source/drain electrodes 833 a and 833 b of the active regions 800 are extended and joined together to form a first source/drain electrode 933 a and a second source/drain electrode 933 b . Thus, a cross-sectional view of the TFT of FIG. 9 taken along a line I-I of FIG. 9 is the same as the cross-sectional view of FIG. 1B and a cross-sectional view of the TFT of FIG. 9 taken along a line II-II of FIG. 9 is the same as the cross-sectional view of FIG. 1C .

In the first active region 900 A, the first source/drain electrode 933 a overlaps partially with a gate electrode 921 and a part of the first source/drain electrode 933 a is offset from the gate electrode 921 . In addition, a second source/drain electrode 933 b overlaps partially with the gate electrode 921 and a part of the second source/drain electrode 933 b is offset from the gate electrode 921 .

As described above with respect to the active region 100 of FIGS. 1A to 1C , in the TFT of FIG. 9 , a high electric field may be prevented from being applied to a channel region by using an offset region separated from the gate electrode 921 by a distance d 9 . Thus, the performance of the TFT may be prevented from degrading, thus improving the “off” current characteristics. In addition, both the first source/drain electrode 933 a and the second source/drain electrode 933 b overlap partially with the gate electrode 921 , thereby improving the “on” current characteristics. Since the first active region 900 A and the second active region 900 B are symmetrical to each other along the horizontal axis of symmetry, the amount of current flowing through a channel region is substantially the same regardless of whether a voltage is applied to the first source/drain electrode 933 a or the second source/drain electrode 933 b , when the magnitude of the voltage is the same for each direction.

Although the first and second source/drain electrodes 933 a and 933 b are connected in the first and second active regions 900 A and 900 B, it is possible to limit a path of current by insulating an active layer 925 .

Various embodiments of a TFT with an offset structure according to the present invention have been described above, but the present invention is not limited to the above embodiments. Features of exemplary embodiments may include both an offset region and an overlapping region being present among the active regions of a cell, as well as electrodes that exhibit some form of symmetry with respect to each other, thereby improving both the “off” current characteristics and the “on” current characteristics.

FIG. 10 is a graph illustrating the results of a simulation in which the characteristics of a channel current I ds (vertical axis) versus a gate voltage V g (horizontal axis) in a TFT according to an embodiment of the present invention were compared with those of comparative examples. In the graph of FIG. 10 , “Conventional TFT⋄” denotes a conventional TFT that does not have an offset structure, “Offset TFT∘” denotes a conventional TFT with an offset structure, and “Hybrid TFTΔ” denotes a TFT with an offset structure, such as illustrated in FIGS. 1A to 1C , according to the present invention. Here, an ATLAS device simulation was used.

Referring to FIG. 10 , a comparison of these three types of TFTs reveals that “Offset TFT∘” shows the smallest amount of “off” current (e.g., corresponding to a negative gate voltage V g ) and also the smallest amount of “on” current (e.g., corresponding to a positive gate voltage V g ). In the conventional TFT “Offset TFT∘” having an offset structure, the amount of “off” current is small due to an offset region, but the amount of “on” current is also small due to a high resistance value of the offset region. “Conventional TFT⋄” shows the largest amount of “on” current but also the largest amount of “off” current due to a leakage current. “Hybrid TFTΔ” shows that the amount of “on” current is as large as that depicted with “Conventional TFT⋄”, but the amount of “off” current is as small as that depicted with “Offset TFT∘”. In the TFT hybrid TFTΔ having an offset structure according to the present invention, it is concluded that the amount of “on” current is large due to an overlapping part of a source/drain electrode and a gate electrode and the amount of “off” current is small due to an offset region between the source/drain electrode and the gate electrode.

That is, it is noted from the graph of FIG. 10 that the amount of “off” current may be reduced while increasing the amount of “on” current, when a TFT having both an offset region and a non-offset region according to an embodiment of the present invention is used. Also, as described above, if source/drain electrodes and the gate electrode (or electrodes) are arranged to be symmetrical (for example, reflectional symmetry, rotational symmetry), the amount of current flowing through a channel region is substantially the same regardless of a direction in which a voltage is applied to a gate when the magnitude of the voltage is the same for each direction.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims, and equivalents thereof.