Method and apparatus for dark current and hot pixel reduction in active pixel image sensors
A method of operating an imager pixel that includes the act of applying a relatively small voltage on the gate of a transfer transistor during a charge acquisition period. If a small positive voltage is applied, a depletion region is created under the transfer transistor gate, which creates a path for dark current electrons to be transferred to a pixel floating diffusion region. The dark electrons are subsequently removed by a pixel reset operation. If a small negative voltage is applied to the transfer gate, electrons that would normally create dark current problems will instead recombine with holes thereby substantially reducing dark current.
1. A method of operating an imager pixel circuit comprising a photosensor, a transfer gate and a floating diffusion region, said method comprising the acts of:
initiating a charge acquisition period for the photosensor;
determining a gain setting associated with the pixel circuit;
applying a substantially zero voltage level to the transfer gate during the acquisition period under a first gain condition; and
applying a non-zero voltage level to the transfer gate during the acquisition period under a second gain condition which represents a higher gain than said first gain condition,
wherein the level of the applied non-zero voltage is one of a plurality of non-zero voltages that is associated with a respective one of a plurality of gain settings.
2. The method of claim 1 , wherein the non-zero voltage level is a positive voltage level.
3. The method of claim 1 , further comprising the act of resetting the floating diffusion region after the acquisition period.
4. The method of claim 1 , wherein the non-zero voltage level is applied during an entirety of the acquisition period.
5. The method of claim 1 , wherein the voltage level is a negative voltage.
6. A method of operating an imager pixel circuit comprising a photosensor, a transfer gate and a floating diffusion region, said method comprising the acts of:
initiating a charge acquisition period for the photosensor;
determining a gain of the imager;
and
applying a positive voltage to the transfer gate during the acquisition periods which serves as a path for dark current electrons to flow to the floating diffusion region,
wherein a level of the applied voltage is dependant upon a determined gain and is:
a substantially zero voltage level if said determined gain is a first gain level, one of a range plurality of non-zero voltage levels that is associated with a respective one of a plurality of gain settings if said determined gain is a second gain level, said second gain level being different from first gain level.
7. The method of claim 6 , further comprising the act of resetting the floating diffusion region after the acquisition period.
8. The method of claim 6 , wherein the voltage level is applied during an entirety of the acquisition period.
9. A method of operating an imager pixel circuit comprising a photosensor, a transfer gate and a floating diffusion region, said method comprising the acts of:
initiating a charge acquisition period for the photosensor;
determining a gain setting associated with the pixel circuit;
and combining dark electrons underneath the transfer gate with electron holes to substantially reduce a number of dark current electrons,
wherein the combining act comprises:
applying a substantially zero voltage to the transfer gate during the acquisition period under a first gain setting,
applying a negative voltage to the transfer gate during the acquisition period under a second gain setting, wherein a level of the applied voltage is one of a plurality of non-zero voltage levels that is associated with a respective one of a plurality of gain settings.
10. The method of claim 9 , wherein the voltage is applied during an entirety of the acquisition period.
11. An imager comprising:
an array of pixels, each pixel comprising a photosensor, a transfer gate and a floating diffusion region; and
a control circuit electrically connected to said array, said control circuit configured to operate each pixel in a selected row by:
initiating a charge acquisition period for the photosensor, and
applying a voltage to the transfer gate of each pixel in the selected row during the acquisition period,
wherein said applied voltage is based on a gain setting of the imager,
wherein a level of the applied voltage is a substantially zero voltage level if said gain setting is a first gain setting,
wherein a level of the applied voltage is one of a plurality of non-zero voltage levels that is associated with a respective one of a plurality of gain settings if said gain setting is a second gain setting different from said first gain setting.
12. The imager of claim 11 , wherein the voltage is a positive voltage.
13. The imager of claim 12 , wherein the positive voltage is greater than approximately 0.0 volts, but no more than approximately 0.8 volts.
14. The imager of claim 11 , wherein said control circuit further controls the pixel by resetting the floating diffusion region after the acquisition period.
15. The imager of claim 11 , wherein the voltage is a negative voltage.
16. The imager of claim 15 , wherein the negative voltage is less than approximately 0.0 volts, but not less than approximately −0.8 volts.
17. The imager of claim 11 , wherein the non-zero voltage is applied during an entirety of the acquisition period.
18. A processor system comprising:
a processor; and
an imager coupled to said processor, said imager comprising:
an array of pixels, each pixel comprising a photosensor, a transfer gate, and a floating diffusion region; and
a control circuit electrically connected to said array,
said control circuit configured to determine a gain setting associated with each pixel, said control circuit configured to operate each pixel in a selected row by initiating a charge acquisition period for the photosensor and applying a voltage to the transfer gate of the pixel in the selected row during the acquisition period,
wherein the applied voltage is:
substantially zero under a first gain condition,
one of a plurality of non-zero voltage levels associated with a respective one of a plurality of gain settings under a second gain condition which represents a higher gain than said first gain condition.
19. The system of claim 18 , wherein the voltage is a positive voltage.
20. The system of claim 19 , wherein the voltage is greater than approximately 0.0 volts, but no more than approximately 0.8 volts.
21. The system of claim 19 , wherein said control circuit is further configured to control the pixel by resetting the floating diffusion region after the acquisition period.
22. The system of claim 18 , wherein the voltage is a negative voltage.
23. The system of claim 22 , wherein the negative voltage is less than approximately 0.0 volts, but not less than approximately −0.8 volts.
24. A method of operating an imager pixel circuit comprising a photosensor, a transfer gate and a floating diffusion region, said method comprising the acts of:
initiating a charge acquisition period for the photosensor;
determining a light intensity associated with the pixel circuit;
applying a substantially zero voltage level to the transfer gate during the acquisition period under a first light intensity; and
applying one of a plurality of non-zero voltage levels to the transfer gate during the acquisition period in accordance with a determined level of light intensity under a second light intensity which represents a higher light intensity than said first light intensity.
25. The method of claim 24 , wherein the voltage is a positive voltage.
26. The method of claim 24 , wherein the voltage is a negative voltage.
27. An imager comprising:
an array of pixels, each pixel comprising a photosensor, a transfer gate, and a floating diffusion region; and
a control circuit electrically connected to said array,
said control circuit configured to determine a light intensity associated with the pixel, said control circuit configured to operate each pixel in a selected row by initiating a charge acquisition period for the photosensor and applying a voltage to the transfer gate of each pixel in the selected row during the acquisition period,
wherein said applied voltage is:
a substantially zero voltage level to the transfer gate during the acquisition period under a first light intensity,
one of a plurality of non-zero voltage levels which corresponds to the detected light intensity on the imager under a second light intensity which represents a higher light intensity than said first light intensity.
28. The imager of claim 27 , wherein the voltage is a positive voltage.
29. The imager of claim 27 , wherein the voltage is a negative voltage.