Pixel with asymmetric transfer gate channel doping

1. A pixel, comprising:

a substrate of a first conductivity type and having a surface;

a photodetector of a second conductivity type that is opposite the first conductivity type;

a floating diffusion region of the second conductivity type;

a transfer region between the photodetector and the floating diffusion;

a gate positioned above the transfer region and partially overlapping the photodetector;

a pinning layer of the first conductivity type extending at least across the photodetector from the gate; and

a channel implant of the first conductivity type extending from between a midpoint of the transfer gate and the floating diffusion to at least across the photodiode and having a dopant concentration such that a dopant concentration of the transfer region is greater proximate to the photodetector than the floating diffusion, and

wherein a peak dopant concentration of the channel implant is at a level and at a depth below the surface such that a partially-buried channel is formed in the transfer region between the photodetector and floating diffusion when the transfer gate is energized.

2. The pixel of claim 1 , wherein the channel implant has a peak dopant concentration at a depth which ranges from approximately 0.03 microns to 0.06 microns below the surface.

3. The pixel of claim 1 , wherein the channel implant has a peak dopant concentration in a range from approximately 1e17/cm 3 to 5e17/cm 3 .

4. The pixel of claim 3 , wherein the channel implant comprises boron.

5. The pixel of claim 1 , wherein the photodetector extends to a depth of at least 0.3 microns below the surface.

6. The pixel of claim 1 , further comprising a deep anti-punchthrough implant of the first conductivity type and having a concentration such that a region of the substrate proximate to floating diffusion has a higher dopant concentration than a region of the substrate proximate to the photodetector.

7. A pixel, comprising:

a substrate of a first conductivity type;

a photodetector of a second conductivity type that is opposite the first conductivity type and configured to convert incident light to a charge;

a floating diffusion of the second conductivity;

a transfer region between the photodetector and floating diffusion;

a gate formed above the transfer region and partially overlapping the photodetector and configured to transfer the charge from the photodetector to the floating diffusion;

a pinning layer of the first conductivity type extending at least across the photodetector from the gate; and a channel implant of the first conductivity type extending from between a midpoint of the transfer gate and the floating diffusion to at least across the photodiode and having a dopant concentration such that a dopant concentration of the transfer region is greater proximate to the photodetector than the floating diffusion, and

wherein a peak dopant concentration of the channel implant is at a level and at a desired depth below the surface such that a surface channel is formed in the transfer region between the photodetector and floating diffusion when the transfer gate is energized.

8. The pixel of claim 7 , wherein the channel implant has a peak dopant concentration which ranges from approximately 8e16/cm 3 to 2e17/cm 3 .

9. The pixel of claim 8 , wherein the channel implant comprises boron.

10. The pixel of claim 8 , wherein desired depth ranges from 0.04 microns to 0.05 microns.

11. The pixel of claim 8 , wherein the photodetector extends to a depth of at least 0.3 microns below a surface of the substrate.

12. The pixel of claim 8 , further including a deep anti-punchthrough implant and a medium anti-punchthrough implant each of the first conductivity type and having dopant concentrations such that a region of the substrate proximate the floating diffusion has a higher dopant concentration than a region of the substrate proximate to the photodetector.

13. A pixel, comprising:

a substrate of a first conductivity type;

a photodetector of a second conductivity type that is opposite the first conductivity type and configured to convert incident light to a charge;

a floating diffusion of the second conductivity;

a transfer region between the photodetector and the floating diffusion;

a gate for transferring the charge from the photodetector to the floating diffusion;

a pinning layer of the first conductivity type extending at least across the photodetector from the gate; and

a channel region of the first conductivity type overlapping with and asymmetrically extending from the transfer region across the photodetector.

14. The pixel of claim 13 , wherein the transfer region forms a surface channel or partially-buried channel when the gate is energized.

15. The pixel of claim 13 , wherein the channel region includes an implant of the first conductivity type having dopant concentrations such that the channel region proximate the floating diffusion has a higher dopant concentration than the channel region proximate to the photodetector.

16. The pixel of claim 15 , wherein the transfer region includes one or more anti-punchthrough implants each of the first conductivity type and having dopant concentrations such that the transfer region proximate the floating diffusion has a higher dopant concentration than the transfer region proximate to the photodetector.

17. The pixel of claim 16 , wherein the one or more anti-punchthrough implants includes a plurality of implants implanted at different depths in the transfer region.