Image sensors having high dynamic range functionalities
An image sensor pixel may include a photodiode, a charge storage region, a floating diffusion node, and a capacitor. A first transistor may be coupled between the photodiode and the charge storage region. A second transistor may be coupled between the charge storage region and the capacitor. The photodiode may generate image signals corresponding to incident light. Multiple image signals may be summed at the charge storage region. The second transistor may determine a portion of the image signal that may be sent to the capacitor for storage. The portion of the image signal that is sent to the capacitor may be a low gain signal. A remaining portion of the image signal may be a high gain signal. The image sensor pixel may also include readout circuitry that is configured to readout low and high gain signals stored at the floating diffusion node in a double-sampling readout operation.
1. An image sensor pixel, comprising:
a photodiode configured to generate charge in response to light;
a floating diffusion region;
a first charge storage structure coupled to a first path between the photodiode and the floating diffusion region;
a second charge storage structure coupled to a second path between the photodiode and the floating diffusion region;
a transistor interposed between the first charge storage structure and the floating diffusion region; and
an additional transistor coupled between the photodiode and the first charge storage structure and configured to set an overflow charge barrier, wherein the first charge storage structure is configured to receive a portion of the generated charge determined by the overflow charge barrier.
2. The image sensor pixel defined in claim 1 , further comprising:
a first transfer transistor coupled between the photodiode and the second charge storage structure; and
a second transfer transistor coupled between the second charge storage structure and the floating diffusion region.
3. The image sensor pixel defined in claim 1 , wherein the first charge storage structure comprises a capacitor, the portion of the generated charge forms a low gain signal, and the transistor is configured to transfer the low gain signal from the capacitor to the floating diffusion region.
4. The image sensor pixel defined in claim 1 , further comprising:
a reset transistor, wherein the reset transistor and the transistor are coupled between a voltage supply source and the floating diffusion region.
5. The image sensor pixel defined in claim 1 , further comprising:
a connection transistor, wherein the connection transistor and the transistor are coupled between the first charge storage structure and the floating diffusion region.
6. The image senor pixel defined in claim 1 , wherein the first charge storage structure comprises a capacitor.
7. The image sensor pixel defined in claim 1 , further comprising:
a third charge storage structure coupled between the photodiode and the first charge storage structure and between the photodiode and the second charge storage structure.
8. The image sensor pixel defined in claim 7 , wherein the third charge storage structure is configured to integrate the charge with additional charge generated by the photodiode.
9. The image sensor pixel defined in claim 1 , wherein the first and second paths includes respective portions that are parallel paths between the photodiode and the floating diffusion region.
10. The image sensor pixel defined in claim 1 , wherein the transistor is interposed between a voltage source and the floating diffusion region.
11. The image sensor pixel defined in claim 1 , wherein the overflow charge barrier comprises a voltage threshold set by the additional transistor.
12. A method of operating an image sensor having an image pixel that includes a photodiode, a floating diffusion region, a charge storage structure interposed between the photodiode and the floating diffusion region, and an additional charge storage structure interposed between the photodiode and the floating diffusion region, the method further comprising:
at the photodiode, generating charge in response to incident light;
at a transistor interposed between the photodiode and the charge storage structure, setting a voltage threshold;
at the charge storage structure, storing a portion of the charge determined by the voltage threshold; and
at the additional charge storage structure, storing a remaining portion of the charge, wherein the portion of the charge comprises a low gain signal, and the remaining portion of the charge comprises a high gain signal.
13. The method defined in claim 12 , further comprising:
generating a high dynamic range image based on the portion of the charge determined by the voltage threshold and the remaining portion of the charge.
14. The method defined in claim 12 , wherein generating charge in response to incident light comprises generating the charge during a plurality of separate integration periods.
15. The method defined in claim 14 , further comprising:
at the additional charge storage structure, summing at least some portions of the charge generated during the plurality of separate integration periods.
16. An image sensor pixel, comprising:
a photosensitive region configured to generate, in response to light, a first charge during a first integration time period and a second charge during a second integration time period;
a first charge transfer transistor configured to transfer the first and second charges from the photosensitive region to a charge storage structure, wherein the charge storage structure is configured to generate a summed charge based on the first and second charges;
a second charge transfer transistor configured to generate an overflow charge based on the summed charge; and
an additional transistor configured to reset the photosensitive region to a reset voltage between the first and second integration time periods.
17. The image sensor pixel defined in claim 16 , further comprising:
a capacitor configured to receive the overflow charge via the second charge transfer transistor.
18. The image sensor pixel defined in claim 17 , wherein the second charge transfer transistor is configured to provide an overflow threshold, and the overflow threshold generates the overflow charge by determining an overflow portion of the summed charge.
19. The image sensor pixel defined in claim 16 , further comprising:
a gain select transistor, wherein the gain select transistor is configured to transfer the overflow charge to a floating diffusion region.