Electronic device capable of detecting images in low-light environment

This non-provisional application claims priority of China patent application No. 201910245219.0, filed on 28 Mar. 2019, included herein by reference in its entirety.

1. Field of the Disclosure

The disclosure relates to electronic devices, and specifically, to an electronic device having a photo-sensing component used to detect images.

2. Description of the Prior Art

Active pixel sensors are image sensors including photodiodes and active amplifiers, and are widely applicable to image sensing in devices such as digital cameras, digital scanners and fingerprint scanners. The active pixel sensors employ photodiodes to detect light intensity and convert the same into current having corresponding magnitudes. In the conventional approach, the area of the photodiodes may be increased to increase the current of the photodiodes in order to detect images when the light intensity is low. However, increasing the area of the photodiodes can also result in increased parasitic capacitance, leading to a reduction in the voltage generated by the photodiodes, and being inefficient for light detection and unable to provide clear images.

An active pixel sensor is in need to detect images in a low-light environment and enhance image quality.

According to one embodiment of the invention, an electronic device includes a photodiode, a first transistor, a second transistor, a third transistor and a capacitor. The photodiode has a first terminal and a second terminal. The first transistor has a control terminal used to receive a reset signal, a first terminal coupled to the second terminal of the photodiode, and a second terminal. The second transistor has a control terminal coupled to the second terminal of the photodiode, a first terminal and a second terminal. The third transistor has a control terminal used to receive a row selection signal, a first terminal coupled to the second terminal of the second transistor, and a second terminal. The capacitor has a first terminal coupled to the second terminal of the photodiode, and a second terminal coupled to the second terminal of the first transistor.

These and other objectives of the present disclosure will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the embodiment that is illustrated in the various figures and drawings.

FIG. 1 is a schematic diagram of an electronic device according to an embodiment of the disclosure.

FIG. 2 is a timing diagram of a control method of the electronic device in FIG. 1 .

FIG. 3 is a timing diagram of another control method of the electronic device of FIG. 1 .

FIG. 4 is a schematic diagram of an electronic device according to another embodiment of the disclosure.

FIG. 5 is a timing diagram of a control method of the electronic device in FIG. 4 .

FIG. 6 is a timing diagram of another control method of the electronic device of FIG. 4 .

FIG. 7 is a schematic diagram of an electronic device according to an embodiment of the disclosure.

FIG. 1 is a schematic diagram of an electronic device 1 according to an embodiment of the disclosure. The electronic device is applicable to a complementary metal oxide semiconductor field-effect transistor (CMOS) image sensor or fingerprint detector, and may comprise an image sensing circuit 10 , a reset driver 12 , a scan driver 14 and a data driver 16 . The reset driver 12 , the scan driver 14 and the data driver 16 are respectively coupled to the image sensing circuit 10 to control the image sensing circuit 10 to operate in a reset mode, a sensing mode or a scan mode.

The image sensing circuit 10 may be an active pixel sensor, and may include a photodiode D 1 , a first transistor M 1 , a second transistor M 2 , a third transistor M 3 and a capacitor Cp. The photodiode D 1 has a first terminal configured to receive a first voltage V 1 , and a second terminal. The photodiode D 1 has a parasitic capacitor Cd. The first transistor M 1 has a control terminal coupled to the reset driver 12 to receive a reset signal Srst, a first terminal coupled to the second terminal of the photodiode D 1 , and a second terminal configured to receive a second voltage V 2 . The second transistor M 2 has a control terminal coupled to second terminal of the photodiode D 1 , a first terminal configured to receive the third voltage V 3 , and a second terminal. The third transistor M 3 has a control terminal coupled to the scan driver 14 to receive a row selection signal Srow, a first terminal coupled to the second terminal of the second transistor M 2 , and a second terminal coupled to the data driver 16 to output an output signal Scol. The capacitor Cp has a first terminal coupled to the second terminal of the photodiode D 1 and the first terminal of the first transistor M 1 , and a second terminal configured to receive the second voltage V 2 . The first voltage V 1 exceeds the second voltage V 2 . The third voltage V 3 may be a variable voltage. The first transistor M 1 , the second transistor M 2 and the third transistor M 3 may all be N-type metal oxide semiconductor (NMOS) transistors. The first transistor M 1 , the second transistor M 2 and the third transistor M 3 may have a dual gate structure to reduce leakage currents, reducing interference of the leakage currents to a photocurrent I D1 generated by the photodiode D 1 . The first transistor M 1 may reset the capacitor Cp. The second transistor M 2 may amplify the current I D1 generated by the photodiode D 1 . The third transistor M 3 may select an output signal Scol to be read.

In the reset mode, the image sensing circuit 10 may receive the row selection signal Srow to turn off the third transistor M 3 , and receive the reset signal Srst to turn on the first transistor M 1 to reset a voltage V A to the second voltage V 2 , so as to reset the capacitor Cp, setting the photodiode D 1 to a reverse biased state and to be ready for detecting light intensity of incident light. A voltage difference between the control terminal and the second terminal of the first transistor M 1 ranges between 0.5V and 2V. A voltage difference between the first terminal and the second terminal of the photodiode D 1 ranges between 0.5V and 1.5V. In the sensing mode, the first transistor M 1 remains off, the photodiode D 1 is in the reverse biased state to convert the incident light into the current I D1 , the reverse bias current I D1 charges the capacitor Cp to pump the voltage V A to (V 2 +Vd), the voltage Vd being a voltage generated by charging the capacitor Cp using the current I D1 . In the scan mode, the third transistor M 3 may be turned on by the row selection signal Srow, the second transistor M 2 may be configured into a source follower to convert the voltage V A into the current Id, so as to output a voltage Vout to serve as the output signal Scol. Then a processor receives the output signal Scol via the data driver 16 to process and analyze the same to obtain light intensity information. The voltage Vout may be expressed by Equation (1):

V ⁢ out = V2 + I D ⁢ ⁢ 1 * t C - V th - 2 ⁢ L * Id W ⁢ ⁢ μ ⁢ ⁢ C ox Equation ⁢ ⁢ ( 1 )

wherein:

V 2 is the second voltage;

I D1 is the current generated by the photodiode D 1 ;

t is the charging time of charging the capacitor Cp;

C is the capacitance of the capacitor Cp;

V th is the threshold voltage of the second transistor M 2 ;

Id is the drain current of the second transistor M 2 ;

L, W, μ, Cox are the channel length, the channel width, the mobility and the oxide capacitance of the second transistor M 2 , respectively.

As indicated in Equation (1), the output voltage Vout is positively correlated to the current I D1 of the photodiode D 1 and the charging time t of charging the capacitor Cp, and are negatively correlated to the capacitance C of the capacitor Cp. The smaller the capacitance C is, the larger the current I D1 is, or the longer the charging time t is, the larger the output voltage Vout will be, resulting in easier low-light image recognition from under the screen. In some embodiments, the area of the photodiode D is increased to increase the reverse bias current I D,r and the output voltage Vout in order to recognize images in a low-light environment, and an equivalent capacitance of the parasitic capacitor Cd and the capacitor Cp is limited to a capacitance C, thereby eliminating the impact of the increased parasitic capacitor Cd without reducing the output voltage Vout. The capacitor Cp may have a fixed capacitance C between 1.2 fF and 10 fF, equivalent to 2% and 20% of the capacitance of the parasitic capacitor Cd.

The first transistor M 1 and the third transistor M 3 may be NMOS transistors or PMOS transistors. The reset driver 12 and the scan driver 14 may adjust the reset signal Srst and the row selection signal Srow in an appropriate manner to control the first transistor M 1 and the third transistor M 3 to perform the operations. In some embodiments, the first transistor M 1 is an NMOS transistor and the third transistor M 3 is a PMOS transistor. In other embodiments, the first transistor M 1 is a PMOS transistor and the third transistor M 3 is an NMOS transistor. In other embodiments, both the first transistor M 1 and the third transistor M 3 are PMOS transistors. Although only one image sensing circuit 10 is shown in FIG. 1 , the electronic device 1 a may include a plurality of image sensing circuits 10 which maybe arranged in an array for sensing light at different positions.

FIGS. 2 and 3 show timing diagrams of selected signals of the electronic device 1 operating by two control methods. The selected signals include reset signals Srst[n], Srst[n+1] and the row selection signals Srow[n], Srow[n+1], respectively representing the reset signals and the row selection signals transmitted to the image sensing circuits 10 on the nth row and the (n+1)th row. The two control methods in FIGS. 2 and 3 may be used to detect light by way of sensing and scanning row by row. The reset signals Srst[n], Srst[n+1] may be used to set the image sensing circuits 10 on the nth row and the (n+1)th row to the reset mode during the reset period Treset, and set the image sensing circuits 10 on the nth row and the (n+1)th row to the sensing mode during the sensing period Tsense. The row selection signals Srow[n], Srow[n+1] may be used to set the image sensing circuits 10 on the nth row and the (n+1)th row to the scan mode during the scan period Tscan. FIG. 2 shows a control method employing synchronous reset signals Srst[n], Srst[n+1] and row selection signals Srow[n], Srow[n+1], performing the sensing mode and the scan mode simultaneously, and reducing a detection time. FIG. 3 shows a control method employing asynchronous reset signals Srst[n], Srst[n+1] and row selection signals Srow[n], Srow[n+1], performing the sensing mode for a period of time prior to performing the scan mode, and generating clearer signals. The operation methods of the reset mode, sensing mode and the scan mode have been provided in the description for FIG. 1 , and will be omitted here for brevity.

In FIG. 2 , the reset signal Srst[n] is set to a high potential VH to reset the voltage V A to the second voltage V 2 , then the reset signal Srst[n] is set to a low potential VL and the row selection signal Srow[n] is set to the high potential VH to simultaneously sense a change of the voltage V A and read an output signal Scol[n] on the nth row. That is, the reset signal Srst[n] and the row selection signal Srow[n] are complementary signals. During performing the sensing mode and the scan mode on the nth row, the reset signal Srst[n+1] is set to the high potential VH, and after completion of the sensing mode and the scan mode of the nth row, the reset signal Srst[n+1] is set to the low potential VL while the row selection signal Srow[n+1] is set to the high potential VH, so as to simultaneously sense the change of the voltage V A and read an output signal Scol[n+1] on the (n+1) row. That is, the reset signal Srst[n+1] and the row selection signal Srow[n+1] are complementary signals. By employing the control method in FIG. 2 , the electronic device 1 may detect the light intensity and read corresponding output signals Scol[n], Scol[n+1] row by row.

The control method in FIG. 3 is similar to that in FIG. 2 , but the sensing period Tsense of the nth row is extended and the scan period Tscan starts when the sensing period Tsense of the nth row is almost completed, and the (n+1) row operates in a similar manner. Compared to FIG. 2 , in FIG. 3 , the sensing period Tsense is longer, the capacitor Cp has a longer charging time, generating a higher voltage V A , and resulting in a larger output signal Scol correspondingly.

FIG. 4 is a schematic diagram of an electronic device 4 according to an embodiment of the disclosure. The electronic device 4 includes an image sensing circuit 40 , a reset driver 42 , the scan driver 14 and the data driver 16 . The reset driver 42 , the scan driver 14 and the data driver 16 are respectively coupled to the image sensing circuit 40 to control the image sensing circuit 40 to operate in the reset mode, the sensing mode or the scan mode. The electronic device 4 is configured and operated in a manner similar to the electronic device 1 , except that the NMOS first transistor M 1 in the image sensing circuit 10 is replaced with a PMOS first transistor M 1 ′ to enable the reset driver 42 to generate a corresponding reset Srst. Therefore, the explanation for the operations of the electronic device 4 will be omitted here.

FIGS. 5 and 6 show timing diagrams of selected signals of the electronic device 4 operated by two control methods. The selected signals include reset signals Srst[n], Srst[n+1] and the row selection signals Srow[n], Srow[n+1], respectively representing the reset signals and the row selection signals transmitted to the image sensing circuits 40 on the nth row and the (n+1)th row. Both the two control methods in FIGS. 5 and 6 may detect light by way of sensing and scanning row by row. FIG. 5 shows a control method employing synchronous reset signals Srst[n], Srst[n+1] and row selection signals Srow[n], Srow[n+1], performing the sensing mode and the scan mode simultaneously, and reducing a detection time. FIG. 6 shows a control method employing asynchronous reset signals Srst[n], Srst[n+1] and row selection signals Srow[n], Srow[n+1], performing the sensing mode for a period of time prior to performing the scan mode, and generating clearer signals. Since the first transistor M 1 ′ is a PMOS transistor, the reset signals Srst[n], Srst[n+1] and the row selection signals Srow[n], Srow[n+1] in FIG. 5 are identical signals and may be configured in the same manner, and as a result, one of the reset driver 42 and the scan driver 14 may be used to generate identical signals for the reset signals Srst[n], Srst[n+1] and the row selection signals Srow[n], Srow[n+1], reducing hardware costs . In other words, the reset signals Srst[n] and the row selection signals Srow[n] are identical signals, and the reset signals Srst[n+1] and the row selection signals Srow[n+1] are identical signals. Correspondingly, the control terminal of the first transistor M 1 ′ and the control terminal of the third transistor M 3 may receive the identical signals generated by the one of the reset driver 42 and the scan driver 14 . In comparison, in FIG. 6 , since the reset signals Srst[n], Srst[n+1] and the row selection signals Srow[n], Srow[n+1] are asynchronous, the sense period Tsense occurs prior to the scan period Tscan and the length of the sense period Tsense exceeds that of the scan period Tscan, the reset driver 42 and the scan driver 14 may separately generate the reset signals Srst[n], Srst[n+1] and the row selection signals Srow[n], Srow[n+1]. FIGS. 5, 6 correspond to the control methods of controlling the electronic device 1 in FIGS. 2, 3 , the details therefor will not be repeated here.

FIG. 7 is a schematic diagram of an electronic device 7 according to another embodiment of the disclosure. The electronic device 7 includes a pixel array 70 , the reset driver 12 , the scan driver 14 and the data driver 16 . The reset driver 12 , the scan driver 14 and the data driver 16 are respectively coupled to the pixel array 70 to control the pixel array 70 to operate in the reset mode, the sensing mode or the scan mode. The pixel array 70 includes a plurality of image sensing circuits 10 ( 1 , 1 ) to 10 (M,N) arranged in an array. The reset driver 12 and the scan driver 14 maybe respectively disposed on both sides of the pixel array 70 , and the data driver 16 may be disposed on the bottom of the pixel array 70 . The reset signal Srst[1] is used to reset the image sensing circuits 10 on the first row, and the reset signal Srst[2] is used to resets the image sensing circuits 10 on the second row. The row selection signal Srow[1] is used to select the image sensing circuits 10 on the first row, and the row selection signal Srow[2] is used to select the image sensing circuits 10 on the second row to read the detected light intensity.

The electronic devices in FIGS. 1, 4 and 7 may increase reverse bias current I D1 by increasing the area of the photodiode D 1 , while coupling the capacitor Cp to the photodiode D in series to limit the equivalent capacitance of the photodiode D 1 and the capacitor Cp to the capacitance C, thereby increasing the output voltage Vout, being easier to detect low-light image when used in a fingerprint scanner.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the disclosure. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.