Conversion gain modulation using charge sharing pixel

Patent metadata

Field	Value
Publication number	US-10129487-B2
Application number	US-201414473218-A
Country	US
Kind code	B2
Filing date	Aug 29, 2014
Priority date	Jan 2, 2011
Publication date	Nov 13, 2018
Grant date	Nov 13, 2018

Abstract

An image sensor including an array of pixel elements is operated according to two operation modes to modulate the conversion gain of the pixel to operate the image sensor based on the impinging light conditions. More specifically, an image sensor pixel element is operated in a high conversion gain mode for low light conditions and in a low conversion gain mode for bright light conditions. The low conversion gain mode implements charge sharing between the floating diffusion and the photodiode. The low conversion gain mode further implements partial reset where the photodiode and the floating diffusion are reset to the same potential and to a potential slightly less than the pinning voltage of the photodiode.

First claim

The invention claimed is:

1. An imaging device comprising:
a photodiode;
a floating diffusion;
a transfer gate transistor configured to electrically connect to the photodiode and the floating diffusion;
a reset transistor configured to electrically connect to the floating diffusion; and
a source follower transistor configured to electrically connect to the floating diffusion,
wherein a drain of the reset transistor is connected to a first voltage or a second voltage via a switch so as to selectively receive either the first voltage or the second voltage based on a plurality of operation modes for a plurality of conversion gains, and a drain of the source follower transistor is connected to the first voltage independent of whether the drain of the reset transistor is connected to the first voltage or the second voltage.

2. The imaging device according to claim 1 wherein:
in a second operation mode of the plurality of operation modes, during a charge transfer operation with the transfer gate transistor turned on, charge collected at the photodiode based on an impinging light equilibrates between the photodiode and the floating diffusion.

3. The imaging device according to claim 1 wherein:
in a first operation mode of the plurality of operation modes, during a reset operation of the photodiode, the photodiode is reset to a pinning voltage and the floating diffusion node is reset to the first voltage.

4. The imaging device according to claim 1 wherein the drain of the reset transistor is configured to be switched between the first voltage and the second voltage in response to a mode select signal.

5. The imaging device according to claim 1 wherein the mode select signal is a signal indicative of the light intensity level impinging on the photodiode.

6. The imaging device according to claim 1 wherein the mode select signal is generated from a user input.

7. The imaging device according to claim 1 wherein:
the photodiode is one of a plurality of photodiodes in an array; and
a first group of the plurality of photodiodes in the array are operated in a first operation mode of the plurality of operating modes and a second group of the plurality of photodiodes in the array are operated in a second operation mode of the plurality of operation modes.

8. The imaging device according to claim 1 , further comprising:
a row select transistor configured to electrically connect to the floating diffusion and a bitline, wherein:
the transfer gate transistor is controlled by a transfer gate signal,
the reset transistor is controlled by a reset signal,
the row select transistor is controlled by a row select signal,
a reset operation is performed by asserting the transfer gate signal and the reset signal simultaneously, and
a data read operation is performed by asserting the row select signal to read out an output pixel voltage.

9. The imaging device according to claim 8 wherein:
in a second operation mode of the plurality of operation modes, the row select signal is asserted following the reset operation to read out a reset voltage of the floating diffusion.

10. The imaging device according to claim 8 wherein:
in a second operation mode of the plurality of operation modes, the transfer gate signal is deasserted during at least a portion of a light integration operation, and the transfer gate signal and the row select signal are asserted at the end of the light integration operation and during the data read operation to read out the output pixel voltage.

11. The imaging device according to claim 8 wherein:
in a second operation mode of the plurality of operation modes, the transfer gate signal is asserted during an entire light integration operation, and the transfer gate signal and the row select signal are asserted during the data read operation to read out the output pixel voltage.

12. The imaging device according to claim 8 , wherein:
in a second operation mode of the plurality of operation modes, the transfer gate signal is deasserted during at least a portion of a light integration operation, the transfer gate signal is asserted near the end of the light integration operation to transfer charge from the photodiode to the floating diffusion, and the transfer gate signal is deasserted while the row select signal is asserted during the data read operation to read out an output pixel voltage.

13. The imaging device according to claim 8 wherein:
in a second operation mode of the plurality of operation modes, the transfer gate signal is asserted during an entire light integration operation, and the row select signal is asserted periodically during the light integration operation to read out multiple samples of the output pixel voltage.

14. The imaging device according to claim 8 wherein:
in a second operation mode of the plurality of operation modes, the transfer gate signal is asserted periodically during a light integration period, the transfer gate signal being asserted with a pulse width long enough to allow a charge transfer between the photodiode and the floating diffusion node to reach steady state, and the row select signal is asserted when the transfer gate signal is asserted to read out multiple samples of the output pixel voltage.

15. The imaging device according to claim 8 wherein:
in a second operation mode of the plurality of operation modes, the transfer gate signal is asserted periodically during a light integration period, the transfer gate signal being asserted with a pulse width long enough to allow a charge transfer between the photodiode and the floating diffusion node to reach steady state, and the row select signal is asserted when the transfer gate signal is deasserted to read out multiple samples of the output pixel voltage.

16. A method in an image sensing device, comprising:
providing a photodiode, a floating diffusion, a transfer gate transistor in electrical connection to the photodiode and the floating diffusion, a reset transistor in electrical connection to the floating diffusion, and a source follower transistor in electrical connection to the floating diffusion; and
connecting a drain of the reset transistor to a first voltage or a second voltage via a switch so as to selectively receive either the first voltage or the second voltage based on a plurality of operation modes for a plurality of conversion gains,
wherein a drain of the source follower transistor is connected to the first voltage independent of whether the drain of the reset transistor is connected to the first voltage or the second voltage.

17. The method according to claim 16 , further comprising:
integrating a charge at the photodiode as a function of an impinging light; and
in a second operation mode of the plurality of operation modes, turning on the transfer gate transistor during a charge transfer operation, such that charge collected at the photodiode from the impinging light equilibrates between the photodiode and the floating diffusion.

18. The method according to claim 16 , further comprising:
in a first operation mode of the plurality of operation modes, resetting the photodiode to the pinning voltage and the floating diffusion to the first voltage.

19. The method according to claim 16 , further comprising:
selectively switching between the first voltage and the second voltage in response to a mode select signal, the mode select signal being a signal indicative of a light intensity level impinging on the photodiode or a signal generated from a user input.

20. The method accor

Assignees

• Pixim Inc
• Sony Corp

Inventors

• Ayers Thomas R
• Brady Frederick Thomas
• Kang Jinsuk
• Hwang Sungin
• Yu Sen

Classifications

• H04N25/76

  Addressed sensors, e.g. MOS or CMOS sensors · CPC title

• H04N25/57

  Control of the dynamic range · CPC title

• H04N25/59

  by controlling the amount of charge storable in the pixel, e.g. modification of the charge conversion ratio of the floating node capacitance · CPC title

• H04N25/534

  depending on the spectral component · CPC title

• H04N23/76 Primary

  by influencing the image signals · CPC title