Pixel cell with multiple photodiodes

1. An apparatus comprising:

a first photodiode configured to generate a first charge in response to light of a first frequency range;

a second photodiode configured to generate a second charge in response to light of a second frequency range;

a barrier layer between the first photodiode and the second photodiode;

a first charge sensing unit;

a second charge sensing unit;

a controller configured to:

transfer a first portion of the first charge from the first photodiode to the first charge sensing unit to obtain a first measurement result;

transfer a second portion of the first charge from the first photodiode to the second charge sensing unit to obtain a second measurement result;

perform a 3D sensing operation based on the first measurement result and the second measurement result;

transfer the second charge from the second photodiode to the second charge sensing unit via the first photodiode to obtain a third measurement result; and

perform a 2D sensing operation based on the third measurement result.

2. The apparatus of claim 1 , further comprising a first polysilicon gate and a second polysilicon gate;

wherein:

the first photodiode and the second photodiode are in a semiconductor substrate having a front side surface;

the semiconductor substrate comprises:

a first drain region configured as the first charge sensing unit,

a first channel region connected between the first drain region and the first photodiode,

a second drain region configured as the second charge sensing unit; and

a second channel region connected between the second drain region and the second photodiode;

and the first polysilicon gate and the second polysilicon gate are on the front side surface and over, respectively, the first channel region and the second channel region.

3. The apparatus of claim 2 , wherein the first photodiode comprises a pinning layer; and

wherein the pinning layer is connected to the first channel region and the second channel region.

4. The apparatus of claim 3 , wherein:

the first polysilicon gate extends over a first region of the first photodiode connected to the first channel region;

the second polysilicon gate extends over a second region of the first photodiode connected to the second channel region; and

the controller is configured to:

transmit, via the first polysilicon gate, a first signal to modulate electrical potentials of the first region of the first photodiode and of the first channel region to transfer the first portion of the first charge to the first drain region; and

transmit, via the second polysilicon gate, a second signal to modulate electrical potentials of the second region of the first photodiode and of the second channel region.

5. The apparatus of claim 4 , further comprising:

a third drain region configured as a third charge sensing unit to measure a third portion of the first charge;

a third channel region connected between the third drain region and the first photodiode; and

a third polysilicon gate on the front side surface and over the third channel region,

wherein the controller is configured to transmit, via the third polysilicon gate, a fourth signal to transfer the third portion of the first charge to the third drain region via the third channel region.

6. The apparatus of claim 5 , further comprising:

a fourth drain region configured as a charge sink to reset at least one of the first photodiode or the second photodiode;

a fourth channel region connected between the fourth drain region and the first photodiode; and

a fourth polysilicon gate on the front side surface and over the fourth channel region to transmit a fifth signal to reset the first photodiode.

7. The apparatus of claim 6 ,

wherein the barrier layer is configured to block the first charge from flowing into the second photodiode; and

wherein the barrier layer is controllable by a sixth signal to control a flow of the second charge from the second photodiode to the first photodiode.

8. The apparatus of claim 1 , wherein the apparatus further comprises an infra-red illuminator to transmit a first infra-red light pulse;

wherein the first photodiode is configured to generate the first charge based on detecting a second infra-red light pulse from reflection of the first infra-red light pulse; and

wherein the controller is configured to:

control the infra-red illuminator to transmit the first infra-red light pulse between a first time and a second time;

stop the transfer of the first portion of the first charge at a third time; start the transfer of the second portion of the first charge at the third time; and

determine a timing of the second infra-red light pulse based on a quantity of the first portion of the first charge, a quantity of the second portion of the first charge, and the third time.

9. The apparatus of claim 8 , wherein the controller is configured to:

determine a timing of the first infra-red light pulse based on at least one of the first time or the second time; and

determine a timing relationship between the first infra-red light pulse and the second infra-red light pulse based on the timing of the first infra-red light pulse and the timing of the second infra-red light pulse;

and

wherein the 3D sensing operation is based on the timing relationship between the first infra-red light pulse and the second infra-red light pulse.

10. The apparatus of claim 9 , wherein the timing relationship between the first infra-red light pulse and the second infra-red light pulse is determined based on a width of the first infra-red light pulse or a width of the second infra-red light pulse.

11. The apparatus of claim 8 , wherein the third time is based on the second time.

12. The apparatus of claim 8 , further comprising a third charge sensing unit;

wherein the controller is configured to:

prior to the infra-red illuminator transmitting the first infra-red light pulse, transfer a third portion of the first charge to the third charge sensing unit;

measure, based on the third portion of the first charge, an ambient infra-red light; and

determine the timing of the second infra-red light pulse based on a first difference between the first portion of the first charge and the third portion of the first charge and based on a second difference between the second portion of the first charge and the third portion of the first charge.

13. The apparatus of claim 12 , wherein the controller is configured to reset the first photodiode after the transfer of the first charge out of the first photodiode.

14. The apparatus of claim 13 , wherein the second photodiode is configured to generate the second charge based on detecting visible light; and

wherein the controller is configured to:

transfer the second charge from the second photodiode to the second charge sensing unit via the reset first photodiode.

15. The apparatus of claim 14 , wherein the controller is configured to obtain the third measurement result of the visible light based on the second charge and a first infra-red crosstalk component introduced by the second infra-red light pulse, the first infra-red crosstalk component being based on the first charge and a ratio between a spectral response of the first photodiode and a spectral response of the second photodiode within a frequency range of the first infra-red light pulse.

16. The apparatus of claim 15 , wherein the controller is configured to generate the measurement of the visible light based on a second infra-red crosstalk component introduced by the ambient infra-red light.

17. The apparatus of claim 16 , wherein the first infra-red crosstalk component and the second infra-red crosstalk component are determined based on a width of the first infra-red light pulse and an integration period in which the second photodiode generates the second charge in response to the visible light.

18. The apparatus of claim 1 , wherein the first charge sensing unit comprises a floating drain; and

wherein the second charge sensing unit comprises a second floating drain.

19. The apparatus of claim 1 , wherein the 3D sensing operation comprises an indirect time-of-flight measurement operation.

20. A method comprising:

enabling a first photodiode to generate a first charge;

enabling a second photodiode to generate a second charge, the first photodiode and the second photodiode being separated by a barrier layer;

transferring a first portion of the first charge from the first photodiode to a first charge sensing unit to obtain a first measurement result;

transferring a second portion of the first charge from the first photodiode to a second charge sensing unit to obtain a second measurement result;

performing a 3D sensing operation based on the first measurement result and the second measurement result;

transferring the second charge from the second photodiode to the second charge sensing unit via the barrier layer and the first photodiode to obtain a third measurement result; and

performing a 2D sensing operation based on the third measurement result.

21. The method of claim 20 , further comprising:

controlling an infra-red illuminator to transmit a first infra-red light pulse between a first time and a second time;

enabling the first photodiode to generate the first charge based on detecting a second infra-red light pulse from reflection of the first infra-red light pulse within an integration period;

stopping the transfer of the first portion of the first charge at a third time;

starting the transfer of the second portion of the first charge at the third time; and

determining a timing of the second infra-red light pulse based on a quantity of the first portion of the first charge, a quantity of the second portion of the first charge, and the third time.

22. The method of claim 21 , further comprising:

determine a timing of the first infra-red light pulse based on at least one of the first time or the second time; and

determine a timing relationship between the first infra-red light pulse and the second infra-red light pulse based on the timing of the first infra-red light pulse and the timing of the second infra-red light pulse, and

wherein the 3D sensing operation is based on the timing relationship between the first infra-red light pulse and the second infra-red light pulse.

23. The method of claim 22 , further comprising:

prior to the infra-red illuminator transmitting the first infra-red light pulse, transferring a third portion of the first charge from the first photodiode to a third charge sensing unit, the third portion of the first charge being generated by the first photodiode in response to an ambient infra-red light; and

determining the timing of the second infra-red light pulse based on a first difference between the first portion of the first charge and the third portion of the first charge and based on a second difference between the second portion of the first charge and the third portion of the first charge.

24. The method of claim 20 , wherein the 3D sensing operation comprises an indirect time-of-flight measurement operation.