Image sensor and method of driving transfer transistor of image sensor
View Patent ↗Provided is a 4-transistor CMOS image in which a driving condition or a pixel structure is changed so that a transfer transistor in a pixel operates in a pinch-off condition during reset and transfer operations in order to reduce dark current and fixed-pattern noise caused by a change in an operation condition of the transfer transistor and inter-pixel characteristic discrepancy. The image sensor includes a photosensitive pixel including a transfer transistor for transferring photon-induced charges created in a photodiode; and a voltage control unit for controlling a turn-on voltage applied to a gate of the transfer transistor to be lower than a floating diffusion node voltage plus the threshold voltage of the transfer transistor during a partial or entire section of a turn-on section of the transfer transistor such that the transfer transistor operates in a pseudo pinch-off mode.
1. A method of driving a transfer transistor of an image sensor, the method comprising:
applying a pseudo pinch-off voltage as a reset voltage to a gate of the transfer transistor in order to reset a photodiode, such that a floating node and a channel of the transfer transistor are separated and charges in a channel of the transfer transistor are not affected by charges in the floating node;
applying a turn-off voltage to the gate of the transfer transistor to block the photodiode from the channel of the transfer transistor during light accumulation; and
applying the pseudo pinch-off voltage a transfer voltage to the gate of the transfer transistor in order to transfer photon-induced charges accumulated in the photodiode, wherein the amount of charge remaining in the photodiode is maintained constant while the portion-induced charges are transferred.
2. The method of claim 1 , further comprising applying a turn-off voltage to the gate of the transfer transistor while the transferred photon-induced charges are read out.
3. The method of claim 1 , wherein the step of applying the pseudo pinch-off voltage as a reset voltage comprises gradually increasing the gate voltage of the transfer transistor from the turn-off voltage to the pseudo pinch-off voltage.
4. The method of claim 1 , wherein the step of applying the pseudo pinch-off voltage as a reset voltage comprises sequentially applying a first pseudo pinch-off voltage at a first voltage level and a second pseudo pinch-off voltage at a second voltage level which is a voltage of a higher level than the first voltage level to the gate of the transfer transistor.
5. The method of claim 1 , wherein the transfer voltage is the same as the pseudo pinch-off voltage.
6. The method of claim 5 , wherein the step of applying the pseudo pinch-off voltage as the transfer voltage comprises of gradually increasing the gate voltage of the transfer transistor from the turn-off voltage to the pseudo pinch-off voltage.
7. The method of claim 5 , wherein the step of applying the pseudo pinch-off voltage as the transfer voltage comprises of sequentially applying a first pseudo pinch-off voltage at a first level and a second pseudo pinch-off voltage at a second voltage level which is a voltage of a higher level than the first voltage level to the gate of the transfer transistor.
8. The method of claim 1 , wherein the pseudo pinch-off voltage is more than a value several hundreds of mV higher than a power supply voltage and less than a value 1.5 times higher than the power supply voltage.
9. The method of claim 2 , wherein the pseudo pinch-off voltage is more than a value several hundreds of mV higher than a power supply voltage and less than a value 1.5 times higher than the power supply voltage.
10. The method of claim 3 , wherein the pseudo pinch-off voltage is more than a value several hundreds of mV higher than a power supply voltage and less than a value 1.5 times higher than the power supply voltage.
11. The method of claim 4 , wherein the pseudo pinch-off voltage is more than a value several hundreds of mV higher than a power supply voltage and less than a value 1.5 times higher than the power supply voltage.
12. The method of claim 5 , wherein the pseudo pinch-off voltage is more than a value several hundreds of mV higher than a power supply voltage and less than a value 1.5 times higher than the power supply voltage.
13. The method of claim 6 , wherein the pseudo pinch-off voltage is more than a value several hundreds of mV higher than a power supply voltage and less than a value 1.5 times higher than the power supply voltage.
14. The method of claim 7 , wherein the pseudo pinch-off voltage is more than a value several hundreds of mV higher than a power supply voltage and less than a value 1.5 times higher than the power supply voltage.
15. A method of driving a transfer transistor and a reset transistor of an image sensor, the method comprising the steps of:
applying a first turn-on voltage to a gate of the transfer transistor and a second turn-on voltage to a gate of the reset transistor, such that a floating diffusion node and a channel of the transfer transistor are separated and charges in a channel of the transfer transistor are not affected by charges in the floating node;
applying a turn-off voltage to the gate of the transfer transistor to block a photodiode from the channel of the transfer transistor during light accumulation; and
applying a transfer voltage to the gate of the transfer transistor in order to transfer photon-induced charges accumulated in the photodiode, wherein transfer transistor operates in a pseudo pinch-off mode and the amount of charge remaining in the photodiode is maintained constant while the photon-induced charges are transferred.
16. The method of claim 15 , wherein the transfer transistor and the reset transistor have different first and second threshold voltages, and the first and second turn-on voltages applied during the reset period are the same level.
17. The method of claim 15 , wherein the applying the transfer voltage comprises of gradually increasing the gate voltage of the transfer transistor from the turn-off voltage to a pseudo pinch-off voltage.
18. The method of claim 15 , wherein the applying the transfer voltage comprises of sequentially applying at least two pseudo pinch-off voltages from a lower level to a higher level to the gate of the transfer transistor.
19. A method of driving a transfer transistor of an image sensor, the method comprising:
applying a pseudo pinch-off voltage to a gate of the transfer transistor in order to reset a photodiode, such that a floating node and a channel of the transfer transistor are separated and charges in a channel of the transfer transistor are not affected by charges in the floating node;
applying a turn-off voltage to the gate of the transfer transistor to block the photodiode from the channel of the transfer transistor during light accumulation; and
applying a transfer voltage to the gate of the transfer transistor in order to transfer photon-induced charges accumulated in the photodiode;
wherein a turn-on voltage applied to the gate of the transfer transistor is lower than a voltage of the floating node plus a threshold voltage of the transfer transistor.
20. The method of claim 19 , wherein the amount of charge remaining in the photodiode is maintained constant when the photodiode is in a transfer state.