Pixel of display

This application claims the benefit of Taiwan application Serial No. 93132191, filed Oct. 22, 2004, the subject matter of which is incorporated herein by reference.

1. Field of the Invention

The invention relates in general to a pixel of display, and more particularly to an organic light emitting diode (OLED) display pixel.

2. Description of the Related Art

Referring to FIG. 1 , a circuit diagram of a conventional organic light emitting diode (OLED) display pixel is shown. OLED pixel 100 comprises a first transistor Q 1 , a second transistor Q 2 , a third transistor Q 3 , a fourth transistor Q 4 , a capacitor C, and an organic light emitting diode. The current driving method of the OLED pixel 100 is disclosed below. The driving circuit (not shown in FIG. 1 ) provides various currents (a constant source current is exemplified here) according to various gray values, so the pixel 100 generates a corresponding voltage Vc across capacitor C according to the data current I of the constant source current output. That is to say, when the current that flows through the first transistor Q 1 is almost equal to the data current I, the voltage Vgs 1 between the gate and the source of the first transistor Q 1 corresponding to the current flowing through the first transistor Q 1 is stored at the capacitor C and becomes a capacitor cross-voltage Vc. Therefore, when the scanning signal S is disabled, the third transistor 03 , the fourth transistor Q 4 and the first transistor Q 1 are all turned off. Given that the first transistor Q 1 and the second transistor Q 2 have the same characteristics, and that the capacitor cross-voltage Vc is almost maintained at the level of the voltage Vgs 1 , the voltage Vgs 2 between the gate and the source of the second transistor Q 2 would be substantially equal to the voltage Vgs 1 , meanwhile, the current flowing through the second transistor Q 2 would be theoretically equal to the data current I.

In practice, shift in the threshold voltage would occur to the transistor after long duration of operation. The shift has much to do with the operation time of the transistor and the volume of the current flowing through. The turn-on duration of the first transistor Q 1 is different from that of the second transistor Q 2 , because the second transistor Q 2 is always turned on. The second transistor Q 2 is turned on no matter the scanning signal S is enabled or disabled, while the first transistor Q 1 is turned on only when the scanning signal S is enabled. Therefore, the shift in the threshold voltage of conduction for the first transistor Q 1 is different from that for the second transistor Q 2 . The voltage Vgs 2 between the gate and the source of the second transistor Q 2 and the voltage Vgs 1 between the gate and the source of the first transistor may both be equal to the capacitor cross-voltage Vc, but the shift in the threshold voltage for the first transistor Q 1 is different from that for the second transistor Q 2 , resulting in different threshold voltages between the second transistor Q 2 and the first transistor Q 1 . Consequently, the volume of the current generated by the second transistor Q 2 would be different from the data current I, preventing the OLED luminance from achieving the corresponding luminance of the data current I.

It is therefore an object of the invention to provide a pixel of display to resolve the problem that under the same gate-source voltage generated when the first transistor and the second transistor have different shifts in threshold voltage, the current generated by the second transistor is different from the current generated by the first transistor.

The invention achieves the above-identified object by providing a pixel of a display, comprising a switch circuit, a first transistor, a second transistor, a switch, a capacitor, and a lighting emitting element. The switch circuit is for receiving a data signal, and providing a first and a second current according to the data signal when a scanning signal is enabled. The first transistor is for receiving the first current and being coupled to a node. The second transistor is for receiving a first constant voltage and being coupled to the node. The switch is for receiving a second constant voltage and is controlled by a controlling signal corresponding to the scanning signal, and the first transistor receives the current from the switch when the scanning signal is disabled. The capacitor is for receiving the second current. The currents outputted from the first and second transistors are corresponding to the voltage across the capacitor. The first current and the second current vary with the voltage across the capacitor. The lighting emitting element is coupled to the node for receiving the current outputted from the first transistor and the second transistor.

It is therefore another object of the invention to provide an organic light emitting diode (OLED) display, comprising a driving circuit and a plurality of pixels. The driving circuit outputs a data signal and a scanning signal. Each of the plurality of pixels comprises a switch circuit, a first transistor, a second transistor, a switch, a capacitor, and a lighting emitting element. The switch circuit is for receiving the data signal, and providing a first and a second current according to the data signal when the scanning signal is enabled. The first transistor is for receiving the first current and being coupled to a node. The second transistor is for receiving a first constant voltage and being coupled to the node. The switch is for receiving a second constant voltage and is controlled by a controlling signal corresponding to the scanning signal, and the first transistor receives the current from the switch when the scanning signal is disabled. The capacitor is for receiving the second current. The currents outputted from the first and second transistors are corresponding to the voltage across the capacitor. The first current and the second current vary with the voltage across the capacitor. The lighting emitting element is coupled to the node for receiving the current outputted from the first transistor and the second transistor.

Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to accompanying drawings.

FIG. 1 a circuit diagram of a conventional organic light emitting diode (OLED) pixel;

FIG. 2 is a circuit diagram of an OLED display according to a first embodiment of the invention;

FIG. 3 is a circuit diagram of an OLED display according to a second embodiment of the invention; and

FIG. 4 is a circuit diagram of an OLED display according to a third embodiment of the invention.

Referring to FIG. 2 , a circuit diagram of an OLED display according to a first embodiment of the invention is shown. OLED display 200 comprises a driving circuit 202 and a plurality of pixels such as a pixel 208 . The driving circuit 202 outputs a data current I′ and a scanning signal S. The pixel 208 comprises a switch circuit 204 , a first transistor Q 1 ′, a second transistor Q 2 ′, a switch SW, a capacitor C′ and a lighting emitting element 206 . The switch circuit 204 has a third transistor Q 3 ′ and a fourth transistor Q 4 ′. Moreover, the first transistor Q 1 ′, the second transistor Q 2 ′, the third transistor Q 3 ′ and the fourth transistor Q 4 ′ are preferably N-type amorphous silicon thin film transistors, and each has a drain, a gate and a source. Their relationship is disclosed below. The third gate G 3 ′ of the third transistor Q 3 ′ is for receiving the scanning signal S, the third drain D 3 ′ of the third transistor Q 3 ′ is used as an input end IN of the switch circuit 204 to receive the data current I′, while the third source S 3 of the third transistor Q 3 ′ is used as a first output end OUT 1 of the switch circuit 204 . The fourth drain D 4 ′ of the fourth transistor Q 4 ′is coupled to the third source S 3 ′, the fourth gate G 4 ′ of the fourth transistor Q 4 ′ is for receiving the scanning signal S, and the fourth source S 4 ′ of the fourth transistor Q 4 ′ is used as a second output end OUT 2 of the switch circuit 204 .

The first drain D 1 ′ of the first transistor Q 1 ′ is coupled to the first output end OUT 1 , the first gate G 1 ′ of the first transistor Q 1 ′ is coupled to the second output end OUT 2 , and the first source S 1 ′ of the first transistor Q 1 ′ is coupled to a node N. The second drain D 2 ′ of the second transistor Q 2 ′ is coupled to the first constant voltage Vcc 1 , the second gate G 2 ′ of the second transistor Q 2 ′ is coupled to the second output end OUT 2 , and the second source S 2 ′ of the second transistor Q 2 ′ is coupled to the node N. The switch SW has a first end X 1 and a second end X 2 , wherein the switch SW is preferably a P-type amorphous silicon thin film transistor, the first end X 1 is coupled to the second constant voltage Vcc 2 , and the second constant voltage Vcc 2 is preferably of the same voltage with the first constant voltage Vcc 1 . The second end X 2 is coupled to the first output end OUT 1 .

One end of the capacitor C′ is coupled to the second output end OUT 2 , another end of the capacitor C′ together with a positive end of a lighting emitting element 206 are coupled to a node N. The lighting emitting element 206 is an organic light emitting diode (OLED).

Firstly, when the scanning signal S is just enabled, most of the data currents I′ flow to the OLED 206 through the fourth transistor Q 4 ′, the capacitor C′ to quickly charge the capacitor C′. When the voltage Vc′ across the capacitor C′ is increased to a level larger than the threshold voltage of the first transistor Q 1 ′, the first transistor Q 1 ′ is turned on, then the data current I′ is flows through the third transistor Q 3 ′ and the fourth transistor Q 4 ′ at the same time. The first current I 1 is outputted at the first output end OUT 1 and the second current I 2 is outputted at the second output end OUT 2 . Since the threshold voltage of the first transistor Q 1 ′ and that of the second transistor Q 2 ′ are substantially the same, when the first transistor Q 1 ′ is turned on, the second transistor Q 2 ′ is turned on as well. When the voltage Vc′ is increased, the volume of the first current I 1 is increased accordingly. Meanwhile, when the second transistor Q 2 ′ is turned on, like the first current I 1 a , the third current I 3 flowing through the second transistor Q 2 ′ is also increased along with the increase in the voltage Vc′. When the increase in the voltage Vc′ suffices the first current I 1 ′ to be almost equal to the data current I′, the voltage between the gate and the source of the first transistor Q 1 ′ is Vgs 1 ′ and the voltage Vc′ is equal to Vgs 1 ′, therefore the voltage between the gate G 1 ′ and the source S 1 ′ of the first transistor Q 1 ′ is maintained at the level of Vgs 1 ′.

Next, when the scanning signal S is disabled, both the third transistor Q 3 ′ and the fourth transistor Q 4 ′ are turned off. Since the controlling signal CS and the scanning signal S are of opposite phases, the scanning signal S is disabled, but the controlling signal CS is enabled so the switch SW is turned on. Meanwhile, since the voltage difference between the gate G 1 ′—the source S 1 ′ of the first transistor Q 1 ′ and the voltage difference between the gate G 2 ′—the source S 2 ′ of the second transistor Q 2 ′ are maintained at the level of the voltage Vc′, the first transistor Q 1 ′ and the second transistor 02 ′ continue to be turned on. Meanwhile, the current of the first current I 1 and that of the third current I 3 respectively flowing through the first transistor Q 1 ′ and the second transistor Q 2 ′ are approximately equal to the current of the data current I′, so the current received by the OLED 206 approximately doubles the data current I′. That is to say, compared with the conventional pixel in FIG. 1 , for the OLED 206 to generate the same luminance, the current that flows through the first current I 1 and the third current I 3 of the first transistor Q 1 ′ and the second transistor Q 2 ′ in the present embodiment is only half of the current that flows through the first transistor Q 1 ′ and the second transistor Q 2 ′ in FIG. 1 . The shift in the threshold voltage of transistor is proportional related to the duration of conduction and the current flowing through the transistor, so the shifting amount of the threshold voltage of the first transistor Q 1 ′ and the second transistor Q 2 ′ in the present embodiment is smaller than that in conventional pixels. Therefore, the lifespan of the pixel 208 in the present embodiment is longer than that of the pixel in FIG. 1 .

Besides, when the first transistor Q 1 ′ is turned on by the switch SW when the scanning signal S is disabled, the operating time of the two transistors Q 1 ′ and Q 2 ′ are almost the same and so are their shifts in threshold voltage after long duration of operation. When the capacitor C′ stores the voltage Vgs 1 ′ between the gate and the source corresponding to the current I 1 flowing through the first transistor Q 1 ′, the voltage Vc′ causes the second transistor Q 2 ′ to generate a current I 3 substantially equal to the current I 1 flowing through the first transistor Q 1 ′. Consequently, target luminance can be generated by the pixel 208 , preventing low luminance problem as would occur in conventional pixels.

Referring to FIG. 3 , a circuit diagram of an OLED display according to a second embodiment of the invention is shown. The OLED display 300 in the present embodiment differs with the OLED display 200 in the first embodiment in that, the two transistors Q 3 ′, Q 4 ′ in the pixel 308 are connected in parallel rather than in serial. That is to say, the fourth drain D 4 ′ of the fourth transistor Q 4 ′ is coupled to the third drain D 3 ′ of the third transistor Q 3 ′. Besides, the second embodiment and the first embodiment have the same effect.

Referring to FIG. 4 , a circuit diagram of an OLED display according to a third embodiment of the invention is shown. The present embodiment differs with the second embodiment in the connection of the capacitor C′. The capacitor C′ has one end coupled to the second output end OUT 2 of the switch circuit 204 and has another end coupled to either the first constant voltage Vcc 1 or the second constant voltage Vcc 2 , as long as the capacitor C′ can store the voltage Vgs 1 ′ between the gate and the source corresponding to the current I 1 ′ of the first transistor Q 1 ′. Besides, the connection between the third transistor Q 3 ′ and the fourth transistor Q 4 ′ can also be changed from a parallel connection to a serial connection.

The pixels disclosed in above embodiments of the invention have a data current lower than that in conventional current driving method. Furthermore, the shift in the threshold voltage of the second transistor and that of the first transistor are almost the same, so that the pixel circuit, according to different driving currents, generates a corresponding voltage between the gate and the source to be used as a voltage the cross capacitor, and that the second transistor generates a third current almost equal to the first current and provides a current substantially doubles the data current for the OLED to achieve the pre-set luminance.

While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.