Active matrix capacitive fingerprint sensor with 2-TFT pixel architecture for display integration

• US 9,946,375 B2
• Filed: 06/30/2015
• Issued: 04/17/2018
• Est. Priority Date: 06/30/2015
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Synaptics Incorporated
• Inventors: Akhavan Fomani, Arash, Smith, Patrick
• Primary Examiner(s): Thomas, Mia M
• Application Number:
  US14/788,499
• Publication Number:
  US 20170003770A1
• Time in Patent Office: 1,022 Days
• CPC Class Codes: G06F 3/041 Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
• CPC Class Codes: G06F 3/0443 using a single layer of sensing electrodes
• CPC Class Codes: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes
• CPC Class Codes: G06V 40/1318 using electro-optical elements or layers, e.g. electroluminescent sensing

Abstract

Embodiments described herein include an input device including an array of sensing pixels configured to sense an input object in a sensing region. Each of the sensing pixels includes a sense element and a first transistor, wherein the first transistor includes a gate terminal connected to a row select line and a second terminal connected to the sense element. Each of the sensing pixels also includes a second transistor, wherein the second transistor includes a gate terminal connected to the sense element and the second terminal of the first transistor, and wherein the second transistor further includes a second terminal connected to a column output line.

Claims

1. An input device, comprising:

an array of sensing pixels configured to sense an input object in a sensing region, each of the sensing pixels comprising;

a sense element;

a first transistor, wherein the first transistor comprises a gate terminal connected to a row select line and a second terminal connected to the sense element;

a second transistor, wherein the second transistor comprises a gate terminal connected to the sense element and the second terminal of the first transistor, and wherein the second transistor further comprises a second terminal connected to a column output line; and

a reference capacitor disposed between the gate terminal of the second transistor and the second terminal of the second transistor; and

a current sensing amplifier circuit connected to the column output line and configured to measure a current representing the input object.

2. The input device of claim 1, wherein the second transistor further comprises a third terminal connected to a column supply line, and wherein the first transistor further comprises a third terminal connected to an enable line.

3. The input device of claim 2, wherein, while a processing system associated with the input device is in a disable step, the row select line is configured to connect a high voltage to the gate of the first transistor and the enable line is configured to connect a negative voltage to the third terminal of the first transistor.

4. The input device of claim 3, wherein, while the processing system is in the disable step, the row select line is configured to connect to a low voltage to disable a sensing pixel of the array of sensing pixels.

5. The input device of claim 1 further comprising:

a first switch configured to connect and disconnect a positive input terminal of the current sensing amplifier circuit to a first bias voltage, and a second switch configured to connect and disconnect the positive input terminal of the current sensing amplifier circuit to a second bias voltage.

6. The input device of claim 5, wherein the first switch and the second switch are configured to switch between the first bias voltage and the second bias voltage when a processing system associated with the input device is in a readout stage.

7. The input device of claim 1, wherein each of the first transistor and the second transistor are thin-film transistors.

8. The input device of claim 1, wherein the array of sensing pixels has a pitch suitable for distinguishing between features of a fingerprint.

9. A processing system configured to operate an array of sensing pixels to capture an image of an input object, comprising:

a first transistor, wherein the first transistor comprises a gate terminal configured to connect to a row select line and a second terminal configured to connect to a sense element;

a second transistor, wherein the second transistor comprises a gate terminal configured to connect to the sense element and the second terminal of the first transistor, and wherein the second transistor further comprises a second terminal configured to connect to a column output line;

a reference capacitor disposed between the gate terminal of the second transistor and the second terminal of the second transistor;

a readout circuit, wherein the readout circuit comprises a current sensing amplifier circuit configured to connect to the column output line and configured to produce a current representing the input object;

a first switch configured to connect and disconnect a positive input terminal of the current sensing amplifier circuit to a first bias voltage; and

a second switch configured to connect and disconnect the positive input terminal of the current sensing amplifier circuit to a second bias voltage;

wherein a driver module having circuitry is configured to;

connect the sense element to an enable line through the first transistor; and

read the current from the second transistor.

10. The processing system of claim 9, wherein the array of sensing pixels has a pitch suitable for distinguishing between features of a fingerprint.

11. The processing system of claim 9, wherein the first switch and the second switch are configured to switch between the first bias voltage and the second bias voltage during a readout stage of the processing system.

12. The processing system of claim 9, wherein the processing system is further configured to capture an image of a fingerprint pattern of the input object.

13. The processing system of claim 9, wherein the readout circuit further comprises a feedback resistor connected to a negative input terminal of the current sensing amplifier circuit.

14. A method for operating an input device, comprising:

asserting a row select line high to set a voltage at a sense element to zero, wherein the row select line is coupled to a gate terminal of a first transistor, and wherein a second terminal of the first transistor is coupled to the sense element;

asserting the row select line low and biasing an enable line to a negative voltage; and

sensing an output current representing an input object from a current sensing amplifier circuit connected to a column output line, wherein the column output line is coupled to a second terminal of a second transistor and a reference capacitor disposed between a gate terminal of the second transistor and the second terminal of the second transistor;

wherein the gate terminal of the second transistor is coupled to the second terminal of the first transistor, and wherein the output current is proportional to a feature of the input object.

15. The method of claim 14, wherein the output current corresponds to a capacitance between the sense element and the input object.

16. The method of claim 14, further comprising, after sensing the output current, biasing the gate terminal of the second transistor to a negative voltage.

17. The method of claim 14, wherein the current sensing amplifier further comprises:

a first switch for connecting and disconnecting a positive input terminal of the current sensing amplifier circuit to a first bias voltage, and a second switch for connecting and disconnecting the positive input terminal of the current sensing amplifier circuit to a second bias voltage.

18. The method of claim 17, further comprising, prior to sensing the output current:

connecting the second bias voltage to the positive input terminal of the current sensing amplifier.