Lag cancellation in CMOS image sensors

• US 7,417,677 B2
• Filed: 08/08/2003
• Issued: 08/26/2008
• Est. Priority Date: 08/08/2003
• Status: Active Grant

• Current Assignee: Aptina Imaging Corporation (ON Semiconductor Corporation)
• Original Assignee: Micron Technology, Inc.
• Inventors: Jerdev, Dmitri, Agranov, Gennadiy A.
• Primary Examiner(s): Ye; Lin
• Assistant Examiner(s): Giles; Nicholas G.
• Application Number:
  US10/636,534
• Publication Number:
  US 20050030402A1
• Time in Patent Office: 1,845 Days
• Field of Search: 348/308, 348/313, 348/312, 348/314, 348/241, 348/243
• US Class Current: 348/308
• CPC Class Codes: H04N 25/626 Reduction of noise due to residual charges remaining after image readout, e.g. to remove ghost images or afterimages
• CPC Class Codes: H04N 25/76 Addressed sensors, e.g. MOS or CMOS sensors

Abstract

A pixel cell with improved lag characteristics without increased noise. The pixel cell according to embodiments of the invention includes a photo-conversion device and a floating diffusion region switchably coupled to the photo-conversion device. The pixel cell includes a reset transistor, which has a first terminal electrically connected to the floating diffusion region and a second terminal switchably coupled to first and second voltage sources. The first voltage source is higher than the second voltage source. The pixel cell operates by returning a potential on the photo-conversion device to a value approximately equal to a value of a potential barrier between the photo-conversion device and the floating diffusion region prior to generating charge in the photo-conversion device.

Claims

1. A method of operating a pixel cell, the method comprising:

setting a potential on a photo-conversion device to a value approximately equal to a value of a potential barrier formed between the photo-conversion device and a floating diffusion region prior to generating charge in the photo-conversion device by applying a first potential to the photo-conversion device and the floating diffusion region which fills the photo-conversion device and the floating diffusion with charge and applying a second potential to the photo-conversion device and the floating diffusion region which removes charge from the photo-conversion device and the floating diffusion region, wherein the first potential is lower than the second potential;

generating charge in response to light incident on the photo-conversion device;

applying a third potential to the floating diffusion region;

reading out the third potential from the floating diffusion region; and

transferring the generated charge from the photo-conversion device to the floating diffusion region subsequent to reading out the third potential.

2. The method of claim 1, wherein the act of setting the potential on the photo-conversion device comprises setting the potential on a pinned photodiode.

3. The method of claim 1, wherein the act of applying the first potential comprises applying a ground potential.

4. The method of claim 1, wherein the act of applying the first potential comprises closing a first switch electrically connected to the first potential, and wherein the act of applying the second potential comprises closing a second switch electrically connected to the second potential.

5. A method of operating a pixel cell, the method comprising:

applying a first potential to a photo-conversion device and a floating diffusion region which fills the photo-conversion device and the floating diffusion with charge;

applying a second potential to the photo-conversion device and the floating diffusion region which removes charge from the photo-conversion device and the floating diffusion region;

wherein the first potential is lower than the second potential;

generating charge in response to light incident on the photo-conversion device during an integration period;

after the integration period, applying a third potential to the floating diffusion region;

reading out the third potential from the floating diffusion region; and

transferring the photo-generated charge from the photo-conversion device to the floating diffusion region subsequent to reading out the third potential; and

reading out the photo-generated charge from the floating diffusion region.

6. The method of claim 5, wherein the act of applying the first potential comprises closing a first switch electrically connected to the first potential, and wherein the act of applying the second potential comprises closing a second switch electrically connected to the second potential.

7. The method of claim 5, wherein the act of applying the first potential comprises applying a ground potential.

8. The method of claim 5, wherein the act of applying the first potential comprises applying the first potential to a pinned photodiode.

9. A method of operating an image sensor comprising an array of pixel cells, the method comprising:

applying a first potential to a photo-conversion device in each pixel cell and a floating diffusion region which fills the photo-conversion device and the floating diffusion with charge in each pixel cell;

applying a second potential to the photo-conversion device in each pixel cell and the floating diffusion region in each pixel cell which removes charge from the photo-conversion device and the floating diffusion region, wherein the first potential is lower than the second potential;

in each pixel cell, generating charge in response to light incident on the photo-conversion device during an integration period;

after the integration period, applying a third potential to the floating diffusion region in each pixel cell; and

reading out the third potential from the floating diffusion region in each pixel cell.

10. The method of claim 9, wherein the act of applying the first potential comprises closing a first switch electrically connected to the first potential, and wherein the act of applying the second potential comprises closing a second switch electrically connected to the second potential.

11. The method of claim 9, wherein the act of applying the first potential comprises applying a ground potential.

12. The method of claim 9, wherein the act of applying the first potential comprises applying the first potential to a pinned photodiode.