Imaging pixels with improved photodiode structures
View Patent ↗A photodiodes may be formed on a substrate such as an imager substrate. The photodiode may include first and second layers in the substrate that form a p-n junction. The first layer may have a first doping type such as p-type doping, whereas the second layer may have a second, opposite doping type such as n-type doping. A counter-doping implant region may be provided that only partially overlaps with the second layer of the photodiode. The counter-doping implant region may have an opposite doping type to the second layer and may have a dopant concentration that is less than the dopant concentration of the second layer. The counter-doping implant region may extend into a third layer of the substrate that may have the same doping type of the second layer but at a lower concentration than the counter-doping implant region.
1. An imaging pixel, comprising:
a substrate;
a first layer of a first doping type in the substrate;
a second layer of a second doping type in the substrate, wherein the first and second layers form a p-n junction and the second layer is directly beneath the first layer;
a third layer of the second doping type in the substrate, wherein the third layer is directly beneath the second layer; and
a counter-doping implant region in the substrate, wherein the counter-doping implant region has a first portion that extends into the second layer and a second portion that extends into the third layer.
2. The imaging pixel defined in claim 1 wherein the first layer of the first doping type comprises a p-type layer.
3. The imaging pixel defined in claim 2 wherein the second layer of the second doping type comprises an n-type layer.
4. The imaging pixel defined in claim 3 wherein the counter-doping implant region comprises a p-type implant region.
5. The imaging pixel defined in claim 4 wherein the n-type layer has a first dopant concentration, wherein the p-type implant region has a second dopant concentration, and wherein the second dopant concentration is less than the first dopant concentration.
6. The imaging pixel defined in claim 5 wherein the p-type implant region is surrounded above, below and on at least two sides by the second layer and the third layer.
7. The imaging pixel defined in claim 6 wherein the third layer of the second doping type has a concentration of n-type dopants that is less than the first dopant concentration.
8. The imaging pixel defined in claim 7 wherein the concentration of n-type dopants in the third layer is less than the second dopant concentration of the p-type implant region.
9. The imaging pixel defined in claim 8 further comprising a p-well in the substrate, wherein the p-type implant region contacts the p-well.
10. A photodiode, comprising:
a substrate;
first and second layers that form a first p-n junction in the substrate;
a third layer in the substrate; and
an implant region of a single doping type in the substrate, wherein the implant region has a first portion that extends into the second layer and a second portion that extends into the third layer and forms a second p-n junction with the third layer in the substrate.
11. The photodiode defined in claim 10 wherein the first layer is doped with p-type dopants, wherein the second layer is doped with n-type dopants, and wherein the implant region is doped with p-type dopants.
12. The photodiode defined in claim 11 wherein the second layer is doped with a first concentration of the n-type dopants, wherein the implant region is doped with a second concentration of the p-type dopants, and wherein the second concentration is less than the first concentration.
13. The photodiode defined in claim 12 wherein the third layer in the substrate is doped with n-type dopants.
14. The photodiode defined in claim 13 wherein the third layer is doped with a third concentration of the n-type dopants that is less than the second concentration of the p-type dopants of the implant region.
15. A system, comprising:
a central processing unit;
memory;
input-output circuitry; and
an imaging device, wherein the imaging device comprises:
a pixel array formed in a substrate; and
a lens that focuses an image on the pixel array, wherein the pixel array includes at least one photodiode comprising:
a p-n junction in the substrate; and
an implant region of a single doping type that only partially overlaps with a portion of the p-n junction and counter-dopes the overlapped portion of the p-n junction.
16. The system defined in claim 15 wherein the p-n junction in the substrate comprises:
a p-type region; and
an n-type region, wherein the implant region partially overlaps with the portion of the n-type region.
17. The system defined in claim 16 wherein the implant region is implanted with p-type dopants.
18. The system defined in claim 17 wherein the implant region has a first dopant concentration, wherein the n-type region has a second dopant concentration that is greater than the first dopant concentration.
19. The system defined in claim 18 wherein the photodiode further comprises:
an n-type layer in the substrate, wherein the implant region partially overlaps with the n-type layer in the substrate.
20. The system defined in claim 19 wherein the n-type layer in the substrate has a third dopant concentration that is less than the first dopant concentration.