Image sensor with photosensitive thin film transistors and dark current compensation

• US 8,289,429 B2
• Filed: 08/09/2010
• Issued: 10/16/2012
• Est. Priority Date: 04/16/2004
• Status: Expired due to Fees

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: den Boer, Willem, Nguyen, Tin, Green, Patrick J.
• Primary Examiner(s): Moe, Aung S
• Assistant Examiner(s): CHEN, CHIA WEI A
• Application Number:
  US12/852,883
• Publication Number:
  US 20100302419A1
• Time in Patent Office: 799 Days
• Field of Search: 348294-324
• US Class Current: 348/302
• CPC Class Codes: H01L 27/14609 Pixel-elements with integrated switching, control, storage or amplification elements scanning details of imagers (circuitry of solid-state image sensors H04N25/00); circuitry of imagers H04N25/70
• CPC Class Codes: H01L 27/1463 Pixel isolation structures
• CPC Class Codes: H01L 27/14643 Photodiode arrays; MOS imagers
• CPC Class Codes: H01L 27/14663 Indirect radiation imagers, e.g. using luminescent members
• CPC Class Codes: H01L 27/14692 Thin film technologies, e.g. amorphous, poly, micro- or nanocrystalline silicon
• CPC Class Codes: H01L 31/0224 Electrodes
• CPC Class Codes: H04N 5/32 Transforming X-rays cameras or camera modules for generating image signals from X-rays H04N23/30; circuitry of SSIS for transforming X-rays into image signals H04N25/30

Abstract

An image sensor array includes image sensors having photo TFTs to generate photocurrent in response to received images. The photo TFTs each have their respective gate electrodes and source electrodes independently biased to reduce the effects of dark current. Storage capacitors are coupled to each photo TFT and discharged upon generation of a photocurrent. Each storage capacitor is coupled to a readout TFT that passes a current from the storage capacitor to a data line. The photo TFT may be disposed above the storage capacitor to increase the exposed surface area of the photo TFT.

Claims

1. An image sensor for sensing a received image, comprising:

a substrate;

a photo TFT disposed on the substrate to generate a photocurrent responsive to the received image, the photo TFT including,

a source electrode coupled to a first bias line,

a gate electrode coupled to a second bias line,

a drain electrode, and

a semiconductor layer coupled to the source and drain electrodes; and

a storage capacitor disposed on the substrate and coupled to the source electrode and drain electrode of the photo TFT, the storage capacitor storing a charge generated by the photocurrent and comprising a stacked capacitor including a top electrode, a bottom electrode and an electrode disposed between the top electrode and the bottom electrode.

2. The image sensor of claim 1 wherein the source electrode and the drain electrode of the photo TFT each include a plurality of extending members, the extending members of the source electrode interdigitated with the extending members of the drain electrode.

3. The image sensor of claim 1, further comprising:

a readout TFT disposed on the substrate and including,

a gate electrode coupled to a select line,

a source electrode coupled to the drain electrode of the photo TFT,

a drain electrode coupled to a data line, and

a semiconductor layer coupled to the source and drain electrodes.

4. The image sensor of claim 3 further comprising:

a passivation layer disposed on the photo TFT and the readout TFT; and

a light shield disposed on the passivation layer substantially above the readout TFT.

5. The image sensor of claim 3 wherein the electrode disposed between the top electrode and the bottom electrode of the stacked capacitor couples to the source electrode of the readout TFT.

6. The image sensor of claim 1 wherein the semiconductor layer includes a n+ layer in contact with the source and drain electrodes.

7. The image sensor of claim 1 wherein the semiconductor layer of the photo TFT includes amorphous silicon.

8. The image sensor of claim 1 wherein the photo TFT includes a gate insulator disposed between the respective gate electrode and semiconductor layer and preventing contact between the gate electrode and the semiconductor layer.

9. The image sensor of claim 1 wherein the gate electrode is selected from a group consisting of Cu, Cr, Al, Ta, and Ti.

10. The image sensor of claim 1 further comprising a passivation layer disposed on the photo TFT.

11. The image sensor of claim 10 wherein the passivation layer is selected from the group consisting of silicon nitride, silicon oxide, and tantalum oxide.

12. The image sensor of claim 1 further comprising a screen disposed adjacent the photo TFT to convert received x-rays to visible light.

13. An image sensor array for sensing a received image, comprising:

a substrate;

a plurality of bias lines disposed on the substrate;

a plurality of data lines disposed on the substrate;

a plurality of select lines disposed on the substrate;

a plurality of photo TFTs disposed on the substrate to generate a photocurrent responsive to the received image, each photo TFT including,

a source electrode coupled to a corresponding first bias line,

a gate electrode coupled to a corresponding second bias line to provide a bias to the gate electrode independent of the bias to the source electrode,

a drain electrode, and

a semiconductor layer coupled to the source and drain electrodes;

a plurality of storage capacitors disposed on the substrate, each storage capacitor coupled to the source electrode and drain electrode of a corresponding photo TFT, each storage capacitor storing a charge generated by the photocurrent and comprising a stacked capacitor including a top electrode, a bottom electrode and an electrode disposed between the top electrode and the bottom electrode; and

a plurality of readout TFTs disposed on the substrate, each readout TFT including,

a gate electrode coupled to a corresponding select line,

a source electrode coupled to the drain electrode of a corresponding photo TFT and coupled to the storage capacitor,

a drain electrode coupled to a corresponding data line, and

a semiconductor layer coupled to the source and drain electrodes,

wherein each readout TFT passes a current to a corresponding data line in response to the discharge of a corresponding storage capacitor.

14. The image sensor array of claim 13 wherein the source electrode and the drain electrode of the photo TFTs each include a plurality of extending members, the extending members of the source electrode interdigitated with the extending members of the drain electrode.

15. The image sensor array of claim 13 wherein the semiconductor layers of each photo TFT and readout TFT include amorphous silicon.

16. The image sensor array of claim 13 wherein the electrode disposed between the top electrode and the bottom electrode of each stacked capacitor couples to the source electrode of a corresponding readout TFT.

17. The image sensor array of claim 13 wherein each photo TFT and readout TFT includes a gate insulator disposed between the respective gate electrode and semiconductor layer and preventing contact between the gate electrode and the semiconductor layer.

18. The image sensor array of claim 13 further comprising a passivation layer disposed on each photo TFT and readout TFT.

19. The image sensor array of claim 18 further comprising a plurality of light shields disposed on the passivation layer substantially above a corresponding readout TFT.

20. The image sensor array of claim 13 further comprising a screen disposed adjacent to the photo TFTs to convert received x-rays to visible light.

21. An image sensor for sensing a received image, comprising:

a substrate;

a photo TFT to generate a photocurrent responsive to the received image, the photo TFT including,

a source electrode coupled to a first bias line, a gate electrode coupled to a second bias line, a drain electrode, and a semiconductor layer coupled to the source and drain electrodes; and

a storage capacitor disposed between the substrate and the photo TFT and coupled to the source electrode and drain electrode of the photo TFT, the storage capacitor storing a charge generated by the photocurrent;

wherein the storage capacitor comprises a stacked capacitor including a top electrode, a bottom electrode and an electrode disposed between the top electrode and the bottom electrode.

22. The image sensor of claim 21 wherein the source electrode and the drain electrode of the photo TFT each include a plurality of extending members, the extending members of the source electrode interdigitated with the extending members of the drain electrode.

23. The image sensor of claim 21 further comprising:

a readout TFT disposed on the substrate and including,

a gate electrode coupled to a select line,

a source electrode coupled to the drain electrode of the photo TFT,

a drain electrode coupled to a data line, and

a semiconductor layer coupled to the source and drain electrodes.