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US-12621581-B2 - Reset gate for photodiode

US12621581B2US 12621581 B2US12621581 B2US 12621581B2US-12621581-B2

Abstract

A buried channel that partially covers a reset gate channel of a pixel for a light sensor is disclosed. The buried channel can lower a potential barrier between a photodiode and the reset gate so that charge can be drained from the photodiode region faster during a reset period. This may result in a shorter reset period that can increase the frame rate of a global shutter.

Inventors

  • David James COLLINS

Assignees

  • SEMICONDUCTOR COMPONENTS INDUSTRIES, LLC

Dates

Publication Date
20260505
Application Date
20240520

Claims (20)

  1. 1 . A light-sensing pixel, comprising: a first channel layer adjacent to a photodiode region, adjacent to a reset-node region, and extending a first channel length to fill a space between the photodiode region and the reset-node region; a reset gate adjacent to a top surface of the first channel layer and extending the first channel length so that a reset-gate voltage applied to the reset gate generates a first conductive channel between the photodiode region and the reset-node region; a photodiode-extension layer adjacent to a bottom surface of the first channel layer opposite the top surface, the photodiode-extension layer extending from the photodiode region by an extension length that is less than the first channel length; a second channel layer extending between the photodiode region and a storage-node region; and an anti-blooming layer adjacent to the second channel layer.
  2. 2 . The light-sensing pixel according to claim 1 , wherein: a reset voltage is applied to the reset-node region during a reset period to drain charge from the photodiode region through the first conductive channel at a drain rate.
  3. 3 . The light-sensing pixel according to claim 2 , wherein the photodiode-extension layer reduces a potential barrier between the photodiode region and the first conductive channel to increase the drain rate.
  4. 4 . The light-sensing pixel according to claim 1 , wherein the first conductive channel is an inversion layer adjacent to the reset gate at the top surface of the first channel layer.
  5. 5 . The light-sensing pixel according to claim 1 , wherein the photodiode region is an n-type semiconductor, the reset-node region is the n-type semiconductor, and the first channel layer is a p-type semiconductor.
  6. 6 . The light-sensing pixel according to claim 1 , wherein the extension length is in a range of ¼ the first channel length to ¾ the first channel length.
  7. 7 . The light-sensing pixel according to claim 1 , wherein the reset gate includes: an insulating oxide layer between the top surface of the first channel layer and an n-type polysilicon gate electrode.
  8. 8 . The light-sensing pixel according to claim 1 , wherein: the second channel layer extends a second-channel length; and the anti-blooming layer extends the second-channel length.
  9. 9 . The light-sensing pixel according to claim 1 , wherein: the photodiode-extension layer and the anti-blooming layer are on a common fabrication layer of the light-sensing pixel.
  10. 10 . The light-sensing pixel according to claim 8 , further comprising: a transfer gate adjacent to the top surface of the second channel layer and extending the second-channel length so that a transfer-gate voltage applied to the transfer gate generates a second conductive channel between the photodiode region and the storage-node region.
  11. 11 . A light-sensing pixel comprising: a photodiode configured to generate charge based on received light; a reset transistor coupled between the photodiode and a reset-node, the reset transistor including: a reset gate; a first channel layer adjacent to the reset gate at a top surface, the first channel layer configured to generate a first conductive channel between the photodiode and the reset-node while a reset-gate voltage is applied to the reset gate; and a buried channel at a bottom surface of the first channel layer opposite the top surface, the buried channel having an extension length that is less than a channel length of the first channel layer in a direction aligned with the first conductive channel; and a transfer transistor coupled between the photodiode and a storage-node, the transfer transistor including: a transfer gate; a second channel layer extending between a photodiode region and a storage-node region; and an anti-blooming channel adjacent to the second channel layer.
  12. 12 . The light-sensing pixel according to claim 11 , wherein: a reset voltage is applied to the reset-node while the reset-gate voltage is applied to the reset gate during a reset period to drain the charge from the photodiode through the first conductive channel at a drain rate.
  13. 13 . The light-sensing pixel according to claim 12 , wherein the buried channel reduces a potential barrier between the photodiode and the reset transistor to increase the drain rate.
  14. 14 . The light-sensing pixel according to claim 11 , wherein the first conductive channel is an inversion layer in the first channel layer.
  15. 15 . The light-sensing pixel according to claim 11 , wherein the reset transistor is an n-type metal oxide semiconductor field-effect transistor.
  16. 16 . The light-sensing pixel according to claim 11 , wherein the extension length is in a range of ¼ the channel length to ¾ the channel length.
  17. 17 . The light-sensing pixel according to claim 11 , wherein: the second channel layer is adjacent to the transfer gate at a top surface of the second channel layer, the second channel layer configured to generate a second conductive channel between the photodiode and the storage-node while a transfer-gate voltage is applied to the transfer gate; and the anti-blooming channel at a bottom surface of the second channel layer opposite the top surface, the anti-blooming channel extending an anti-blooming channel length that is longer than the extension length.
  18. 18 . The light-sensing pixel according to claim 17 , wherein: the buried channel and the anti-blooming channel are fabrication on a common layer of the light-sensing pixel.
  19. 19 . A method for resetting a light-sensing pixel, the method comprising: generating a first charge in a photodiode region of the light-sensing pixel; applying a reset voltage to a reset-node region of the light-sensing pixel, the reset-node region separated from the photodiode region by a first channel layer, the light-sensing pixel including a photo-diode extension layer partially covering a bottom surface of the first channel layer; applying a reset-gate voltage to a reset gate adjacent to the first channel layer; generating a conductive channel between the photodiode region and the reset-node region in response to the reset-gate voltage; draining the first charge from the photodiode region through the conductive channel to reset the light-sensing pixel; accumulating a second charge in the photodiode region of the light-sensing pixel during an exposure period; and applying a voltage to a transfer gate on a top surface of a second channel layer to couple the second charge to a storage node, the transfer gate extending a second channel length between the photodiode region and a storage-node region, and the transfer gate including an anti-blooming layer adjacent to a bottom surface of the second channel layer and extending the second channel length.
  20. 20 . The method according to claim 19 , wherein the photo-diode extension layer partially covering the bottom surface of the first channel layer reduces a barrier potential to decrease a time required to reset the light-sensing pixel.

Description

FIELD OF THE DISCLOSURE The present disclosure relates to an image sensor and more specifically to a light-sensing pixel for the image sensor. BACKGROUND An image sensor includes a pixel array that can be implemented as photodiodes arranged in rows and columns. Each photodiode is configured to measure photons during an exposure (i.e., frame). The photodiodes may be addressed row-by-row to capture an image in what is known as a rolling shutter. One problem with the rolling shutter is that objects that move faster than the frame rate can appear distorted in images captured by the pixel array. In other words, a fast-moving object may change positions in the time it takes to scan from the first row to the last row. To eliminate this effect, a global shutter may be used. In a global shutter image sensor, all photodiodes of the pixel array are exposed simultaneously before reading them out, and after a readout process, all photodiodes are reset before exposing the next frame. While being able to accurately image a fast-moving object, a global shutter can reduce the frame rate of the image sensor. SUMMARY The present disclosure describes a light-sensing pixel that can help to increase the frame rate of a global shutter image sensor based on its reduced potential barrier between a photodiode and a reset gate, which can make resetting the pixel between frames faster. In some aspects, the techniques described herein relate to a light-sensing pixel, including: a channel layer adjacent to a photodiode region, adjacent to a reset-node region, and extending a channel length to fill a space (i.e., area, volume) between the photodiode region and the reset-node region; a reset gate adjacent to a top surface of the channel layer and extending the channel length so that a reset-gate voltage applied to the reset gate generates a conductive channel between the photodiode region and the reset-node region; and a photodiode-extension layer adjacent to a bottom surface of the channel layer opposite the top surface, the photodiode-extension layer extending from the photodiode region by an extension length that is less than the channel length. In some aspects, the techniques described herein relate to a light-sensing pixel including: a photodiode configured to generate charge based on received light; and a reset transistor coupled between the photodiode and a reset-node, the reset transistor including: a reset gate; a channel layer adjacent to the reset gate at a top surface, the channel layer configured to generate a conductive channel between the photodiode and the reset-node while a reset-gate voltage is applied to the reset gate; and a buried channel at a bottom surface of the channel layer opposite the top surface, the buried channel having an extension length that is less than a channel length of the channel layer in a direction aligned with the conductive channel. In some aspects, the techniques described herein relate to a method for resetting a light-sensing pixel, the method including: generating charge in a photodiode region of the light-sensing pixel; applying a reset voltage to a reset-node region of the light-sensing pixel, the reset-node region separated from the photodiode region by a channel layer, the light-sensing pixel including a photo-diode extension layer partially covering a bottom surface of the channel layer; applying a reset-gate voltage to a reset gate adjacent to the channel layer; generating a conductive channel between the photodiode region and the reset-node region in response to the reset-gate voltage; and draining the charge from the photodiode region through the conductive channel to reset the light-sensing pixel. The foregoing illustrative summary, as well as other exemplary objectives and/or advantages of the disclosure, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of an illustrative imaging and response system including an imaging system that uses an image sensor to capture images. FIG. 2 is an example of an arrangement of the image sensor shown in FIG. 3. FIG. 3 is a schematic block diagram of an image sensor according to a possible implementation of the present disclosure. FIG. 4 is a graph showing the timing of a frame for an image sensor according to a possible implementation of the present disclosure. FIG. 5 is schematic of a light-sensing pixel for an image sensor according to a possible implementation of the present disclosure. FIG. 6 is a top view of a light-sensing pixel according to a possible implementation of the present disclosure. FIG. 7A is a cross-sectional view of a transfer-gate portion of a light-sensing pixel according to a possible implementation of the present disclosure. FIG. 7B is a cross-sectional view of a reset-gate portion of a light-sensing pixel according to a possible implementation of the present disclosure. FIG.