Imaging pixels with a fully depleted charge transfer path

1. An imaging pixel comprising:

a pinned photodiode with a first potential;

a pinned transfer diode with a second potential, wherein the second potential is higher than the first potential;

a floating diffusion region with a third potential, wherein the third potential is higher than the second potential;

a first transfer transistor coupled between the pinned photodiode and the pinned transfer diode; and

a second transfer transistor coupled between the pinned transfer diode and the floating diffusion region, wherein the first transfer transistor, the pinned transfer diode, and the second transfer transistor form a fully depleted charge transfer path between the pinned photodiode and the floating diffusion region.

2. The imaging pixel defined in claim 1 , wherein the first transfer transistor has a circular gate that laterally surrounds the pinned transfer diode.

3. The imaging pixel defined in claim 1 , further comprising:

a plurality of additional pinned photodiodes; and

a plurality of additional transfer transistors, wherein each transfer transistor of the plurality of additional transfer transistors is coupled between a respective pinned photodiode of the plurality of additional pinned photodiodes and the pinned transfer diode.

4. The imaging pixel defined in claim 1 , wherein the pinned photodiode and pinned transfer diode are formed in an upper substrate, wherein the floating diffusion region is formed in a lower substrate, and wherein the imaging pixel further comprises an interconnect layer that couples the upper substrate to the lower substrate.

5. The imaging pixel defined in claim 4 , wherein the interconnect layer is formed in an opaque dielectric layer.

6. The imaging pixel defined in claim 4 , further comprising:

an additional pinned transfer diode that is formed in the lower substrate, wherein the interconnect layer couples the pinned transfer diode in the upper substrate to the additional pinned transfer diode in the lower substrate.

7. The imaging pixel defined in claim 6 , wherein the interconnect layer comprises a metal layer that shields the additional pinned transfer diode and the floating diffusion region from incident light, and wherein the metal layer acts as a reflector for incident light.

8. The imaging pixel defined in claim 6 , wherein the additional pinned transfer diode has a fourth potential that is higher than the second potential and lower than the third potential.

9. The imaging pixel defined in claim 8 , further comprising:

a storage diode with a fifth potential, wherein the fifth potential is higher than the fourth potential and lower than the third potential, wherein the second transfer transistor is coupled between the additional pinned transfer diode and the storage diode; and

a third transfer transistor coupled between the storage diode and the floating diffusion region.

10. The imaging pixel defined in claim 9 , wherein the first transfer transistor is configured to transfer charge from the pinned photodiode to the pinned transfer diode when asserted, wherein the second transfer transistor is configured to transfer charge from the additional pinned transfer diode to the storage diode when asserted, and wherein the third transfer transistor is configured to transfer charge from the storage diode to the floating diffusion region when asserted.

11. The imaging pixel defined in claim 9 , wherein the storage diode is a first storage diode, the imaging pixel further comprising:

a second storage diode;

a fourth transfer transistor coupled between the additional pinned transfer diode and the second storage diode; and

a fifth transfer transistor coupled between the second storage diode and the floating diffusion region.

12. The imaging pixel defined in claim 11 , further comprising:

a third storage diode;

a sixth transfer transistor coupled between the additional pinned transfer diode and the third storage diode; and

a seventh transfer transistor coupled between the third storage diode and the floating diffusion region.

13. The imaging pixel defined in claim 11 , wherein the first and second storage diodes have different sizes.

14. The imaging pixel defined in claim 11 , further comprising:

at least one additional pinned photodiode that is configured to share the pinned transfer diode with the pinned photodiode.

15. A method of operating an imaging pixel, wherein the imaging pixel comprises a pinned photodiode, a first pinned transfer diode, a second pinned transfer diode, an interconnect layer that couples the first pinned transfer diode to the second pinned transfer diode, a storage diode, a floating diffusion region, a first transfer transistor coupled between the pinned photodiode and the first pinned transfer diode, a second transfer transistor coupled between the second pinned transfer diode and the storage diode, a third transfer transistor coupled between the storage diode and the floating diffusion region, a reset transistor that is coupled between the floating diffusion region and a bias voltage, and a shutter gate that is coupled between the second pinned transfer diode and the bias voltage, the method comprising:

resetting the pinned photodiode, the first pinned transfer diode, the second pinned transfer diode, the storage diode, and the floating diffusion region by asserting the first transfer transistor, the third transfer transistor, the shutter gate, and the reset transistor; and

with the photodiode, accumulating charge over a total exposure time that is split into multiple integration periods, wherein the multiple integration periods are non-continuous and are each shorter than the total exposure time, wherein the charge that is accumulated in each integration period of the multiple integration periods is transferred to the storage diode.

16. The method defined in claim 15 , the method further comprising:

after each integration period of the multiple integration periods, clearing charge from the pinned photodiode, the first pinned transfer diode, and the second pinned transfer diode by asserting the first transfer transistor and the shutter gate.

17. A method of operating an imaging pixel, wherein the imaging pixel comprises a pinned photodiode, a first pinned transfer diode, a second pinned transfer diode, an interconnect layer that couples the first pinned transfer diode to the second pinned transfer diode, a first storage diode, a second storage diode, and a third storage diode, a floating diffusion region, a first transfer transistor coupled between the pinned photodiode and the first pinned transfer diode, a second transfer transistor coupled between the second pinned transfer diode and the first storage diode, a third transfer transistor coupled between the first storage diode and the floating diffusion region, a fourth transfer transistor coupled between the second pinned transfer diode and the second storage diode, a fifth transfer transistor coupled between the second storage diode and the floating diffusion region, a sixth transfer transistor coupled between the second pinned transfer diode and the third storage diode, a seventh transfer transistor coupled between the third storage diode and the floating diffusion region, a reset transistor that is coupled between the floating diffusion region and a bias voltage, and a shutter gate that is coupled between the second pinned transfer diode and the bias voltage, the method comprising:

resetting the pinned photodiode, the first pinned transfer diode, the second pinned transfer diode, the first storage diode, the second storage diode, the third storage diode, and the floating diffusion region by asserting the first transfer transistor, the third transfer transistor, the fifth transfer transistor, the seventh transistor, the shutter gate, and the reset transistor;

accumulating a first amount of charge in the photodiode over a first integration time;

transferring the first amount of charge to the first storage diode by asserting the first transfer transistor and the second transfer transistor;

accumulating a second amount of charge in the photodiode over a second integration time;

transferring the second amount of charge to the second storage diode by asserting the first transfer transistor and the fourth transfer transistor;

accumulating a third amount of charge in the photodiode over a third integration time; and

transferring the third amount of charge to the third storage diode by asserting the first transfer transistor and the sixth transfer transistor.

18. The method defined in claim 17 , wherein the first integration time is longer than the second integration time, and wherein the second integration time is longer than the third integration time.

19. The method defined in claim 17 , further comprising:

reading out the charge on the first storage diode using correlated double sampling;

reading out the charge on the second storage diode using correlated double sampling; and

reading out the charge on the third storage diode using correlated double sampling.

20. The method defined in claim 19 , further comprising:

after transferring the third amount of charge to the third storage diode and before reading out the charge on the first storage diode:

accumulating a fourth amount of charge in the photodiode over a fourth integration time; and

transferring the fourth amount of charge to the first storage diode by asserting the first transfer transistor and the second transfer transistor.