Image sensors operable in global shutter mode and having small pixels with high well capacity
View Patent ↗An image sensor operable in global shutter mode ma include small pixels with high charge storage capacity, low dark current, and no image lag. Storage capacity of a photodiode and a charge storage diode may be increased by placing a p+ type doped layer under the photodiode and the charge storage diode. The p+ type doped layer ma include an opening for allowing photo-generated charge carriers to flow from the silicon bulk to the charge storage well located near the surface of the photodiode. A compensating n− type doped implant may be formed in the opening. Image lag is prevented by placing a p− type doped region under the p+ type doped photodiode pinning layer and aligned with the opening. The p+ type doped layer may extend under the entire pixel array.
1. An image sensor pixel in a pixel array having a silicon substrate, comprising:
a photodiode and a charge storage diode formed in a surface of the silicon substrate, wherein the silicon substrate includes a bulk portion under the photodiode and the charge storage diode;
a p+ type doped layer that extends under the photodiode and the charge storage diode parallel to the surface, wherein the p+ type doped layer comprises an opening through which charge carriers pass from the bulk portion of the silicon substrate to the photodiode; and
a compensating n-type doped implant region in the opening.
2. The image sensor pixel defined in claim 1 further comprising a p-type doped implant region interposed between the photodiode and the compensating n-type doped implant region, wherein the p-type doped implant region is aligned with the compensating n-type doped implant region.
3. The image sensor pixel defined in claim 2 wherein the p-type doped layer is connected to ground biased layers.
4. The image sensor pixel defined in claim 2 further comprising a transfer gate formed on the surface of the silicon substrate, wherein the transfer gate transfers charge carriers from the photodiode to the charge storage diode.
5. The image sensor pixel defined in claim 4 further comprising a floating diffusion node.
6. The image sensor pixel defined in claim 5 further comprising an additional transfer gate formed on the surface of the silicon substrate, wherein the additional transfer gate transfers the charge carriers from the charge storage diode to the floating diffusion node.
7. The image sensor pixel defined in claim 4 wherein the transfer gate is shared with an adjacent image sensor pixel.
8. The image sensor pixel defined in claim 2 further comprising a plurality of n− type doped regions formed in the bulk portion of the silicon substrate, wherein the plurality of n− type doped regions form a potential gradient that directs charge carriers from the bulk portion of the silicon substrate through the opening.
9. The image sensor pixel defined in claim 2 further comprising a plurality of pixel separation implants that separate the image sensor pixel from other image sensor pixels in the pixel array.
10. The image sensor pixel defined in claim 1 wherein the image sensor pixel is a backside illuminated image sensor pixel operable in global shutter mode.
11. An image sensor having an array of image sensor pixels and a silicon substrate, the image sensor comprising:
a plurality of photodiodes and a plurality of charge storage diodes formed in a surface of the silicon substrate, wherein the silicon substrate includes a bulk portion under the plurality of photodiodes and the plurality of charge storage diodes;
a p+ type doped layer that extends under the plurality of photodiodes and under the plurality of charge storage diodes parallel to the surface, wherein the p+ type doped layer comprises a plurality of openings through which charge carriers pass from the bulk portion of the silicon substrate to the photodiodes; and
a plurality of compensating n− type doped implant regions, wherein each compensating n− type doped implant region is formed in a respective one of the openings.
12. The image sensor defined in claim 11 further comprising a plurality of p− type doped implant regions, wherein each p− type doped implant region is interposed between a respective one of the photodiodes and a respective one of the compensating n-type doped implant regions, wherein each p− type doped implant region is aligned with the respective compensating n− type doped implant region.
13. The image sensor defined in claim 12 wherein the p-type doped layer is connected to ground biased layers.
14. The image sensor defined in claim 12 further comprising:
a plurality of first transfer gates formed on the surface of the silicon substrate;
a plurality of second transfer gates formed on the surface of the silicon substrate; and
a plurality of floating diffusion nodes formed in the surface of the silicon substrate, wherein each of the first transfer gates transfers charge carriers from a respective one of the photodiodes to a respective one of the charge storage diodes, and wherein each of the second transfer gates transfers the charge carriers from an associated one of the charge storage diodes to an associated one of the floating diffusion nodes.
15. A system, comprising:
a central processing unit;
memory;
input-output circuitry; and
an image sensor pixel in a pixel array having a silicon substrate, comprising:
a photodiode and a charge storage diode formed in a surface of the silicon substrate, wherein the silicon substrate includes a bulk portion under the photodiode and the charge storage diode;
a p+ type doped layer that extends under the photodiode and under the charge storage diode parallel to the surface, wherein the p+ type doped layer comprises an opening through which charge carriers pass from the bulk portion of the silicon substrate to the photodiode; and
a compensating n-type doped implant region in the opening.
16. The system defined in claim 15 further comprising a p-type doped implant region interposed between the photodiode and the compensating n-type doped implant region, wherein the p− type doped implant region is aligned with the compensating n-type doped implant region.
17. The system defined in claim 16 wherein the p− type doped layer is connected to ground biased layers.
18. The system defined in claim 16 wherein the image sensor pixel further comprises a transfer gate formed on the surface of the silicon substrate, wherein the transfer gate transfers charge carriers from the photodiode to the charge storage diode.
19. The system defined in claim 18 wherein the image sensor pixel further comprises a floating diffusion node.
20. The system defined in claim 19 wherein the image sensor pixel further comprises an additional transfer gate formed on the surface of the silicon substrate, wherein the additional transfer gate transfers the charge carriers from the charge storage diode to the floating diffusion node.