Solid-state imaging element, imaging device, and method for controlling solid-state imaging element

1. A solid-state imaging element, comprising:

a photoelectric conversion element;

a front-stage charge holding region;

a rear-stage charge holding region, wherein a capacity of the rear-stage charge holding region is different from a capacity of the front-stage charge holding region;

a front-stage transfer transistor configured to transfer a first charge from the photoelectric conversion element to the front-stage charge holding region and the rear-stage charge holding region;

a rear-stage transfer transistor configured to transfer the first charge from the rear-stage charge holding region to a floating diffusion region;

an intermediate transfer transistor configured to transfer a second charge, which remains in the front-stage charge holding region after the first charge has been transferred from the rear-stage charge holding region to the floating diffusion region, to the floating diffusion region via the front-stage charge holding region;

a light-shielding wall configured to prevent the first charge to leak from the photoelectric conversion element to the rear-stage charge holding region; and

a signal processing circuit configured to:

compare, between a first pixel signal corresponding to an amount of charge transferred from the rear-stage charge holding region and a second pixel signal corresponding to an amount of charge transferred from the front-stage charge holding region and the rear-stage charge holding region, the first pixel signal with a specific threshold; and

select one of the first pixel signal or the second pixel signal based on a comparison result.

2. The solid-state imaging element according to claim 1 , wherein

the front-stage charge holding region and the rear-stage charge holding region are impurity diffusion regions,

a polarity of the front-stage charge holding region is same as a polarity of the rear-stage charge holding region,

a specific impurity diffusion region is between the front-stage charge holding region and the rear-stage charge holding region, and

a polarity of the specific impurity diffusion region is different from the polarity of the front-stage charge holding region and the rear-stage charge holding region.

3. The solid-state imaging element according to claim 1 , wherein the front-stage charge holding region and the rear-stage charge holding region are in a same impurity diffusion region.

4. The solid-state imaging element according to claim 3 , wherein an impurity concentration in a specific region between the front-stage charge holding region and the rear-stage charge holding region in the same impurity diffusion region is different from an impurity concentration in a vicinity of the specific region.

5. The solid-state imaging element according to claim 3 , further comprising an adjustment transistor configured to adjust a height of a potential barrier between the front-stage charge holding region and the rear-stage charge holding region.

6. The solid-state imaging element according to claim 1 , further comprising a vertical scanning circuit configured to:

control each of the front-stage transfer transistor, the rear-stage transfer transistor, and the intermediate transfer transistor to be turned on or off;

turn the front-stage transfer transistor and the intermediate transfer transistor on while the rear-stage transfer transistor is off to transfer the first charge to the front-stage charge holding region and the rear-stage charge holding region;

turn the rear-stage transfer transistor on while the front-stage transfer transistor and the intermediate transfer transistor are off to transfer the charge from the rear-stage charge holding region to the floating diffusion region; and

turn the intermediate transfer transistor and the rear-stage transfer transistor on to transfer the first charge from the front-stage charge holding region to the floating diffusion region.

7. The solid-state imaging element according to claim 1 , further comprising a specific semiconductor substrate, wherein the photoelectric conversion element is on a wired front surface of both surfaces of the specific semiconductor substrate.

8. The solid-state imaging element according to claim 1 , further comprising a specific semiconductor substrate, wherein the photoelectric conversion element is on a back surface of a wired front surface of both surfaces of the specific semiconductor substrate.

9. The solid-state imaging element according to claim 1 , wherein

the photoelectric conversion element includes a first photoelectric conversion element and a second photoelectric conversion element,

the front-stage charge holding region includes a first front-stage charge holding region and a second front-stage charge holding region,

the rear-stage charge holding region includes a first rear-stage charge holding region and a second rear-stage charge holding region,

the front-stage transfer transistor includes a first front-stage transfer transistor and a second front-stage transfer transistor,

the intermediate transfer transistor includes a first intermediate transfer transistor and a second intermediate transfer transistor, and

the rear-stage transfer transistor includes a first rear-stage transfer transistor and a second rear-stage transfer transistor.

10. The solid-state imaging element according to claim 1 , further comprising:

a charge discharge transistor configured to discharge the first charge from the photoelectric conversion element;

a reset transistor configured to initialize the floating diffusion region;

an amplification transistor configured to amplify a signal corresponding to an amount of charge transferred to the floating diffusion region; and

a selection transistor configured to output the amplified signal as a pixel signal based on a specific selection signal.

11. An imaging device, comprising:

a photoelectric conversion element;

a front-stage charge holding region;

a rear-stage charge holding region, wherein a capacity of the rear-stage charge holding region is different from a capacity of the front-stage charge holding region;

a front-stage transfer transistor configured to transfer a first charge from the photoelectric conversion element to the front-stage charge holding region and the rear-stage charge holding region;

a rear-stage transfer transistor configured to transfer the first charge from the rear-stage charge holding region to a floating diffusion region;

an intermediate transfer transistor configured to transfer a second charge, which remains in the front-stage charge holding region after the first charge has been transferred from the rear-stage charge holding region to the floating diffusion region, to the floating diffusion region via the front-stage charge holding region;

a light-shielding wall configured to prevent the first charge to leak from the photoelectric conversion element to the rear-stage charge holding region; and

a signal processing circuit configured to:

compare, between a first pixel signal corresponding to an amount of charge transferred from the rear-stage charge holding region and a second pixel signal corresponding to an amount of charge transferred from the front-stage charge holding region and the rear-stage charge holding region, the first pixel signal with a specific threshold;

select one of the first pixel signal or the second pixel signal based on a comparison result; and

process the selected one of the first pixel signal or the second pixel signal.

12. A method, comprising:

in a solid-state imaging element comprising:

transferring a first charge from a photoelectric conversion element to a front-stage charge holding region and a rear-stage charge holding region which have different capacities;

transferring the first charge from the rear-stage charge holding region in which leakage of the first charge to the photoelectric conversion element is prevented by a light-shielding wall to a floating diffusion region;

transferring a second charge, which remains in the front-stage charge holding region after the first charge has been transferred from the rear-stage charge holding region to the floating diffusion region, to the floating diffusion region via the front-stage charge holding region;

comparing, between a first pixel signal corresponding to an amount of charge transferred from the rear-stage charge holding region and a second pixel signal corresponding to an amount of charge transferred from the front-stage charge holding region and the rear-stage charge holding region, the first pixel signal with a specific threshold; and

selecting one of the first pixel signal or the second pixel signal based on a comparison result.