Active matrix capacitive fingerprint sensor for display integration based on charge sensing by a 2-TFT pixel architecture

• US 10,325,131 B2
• Filed: 06/30/2015
• Issued: 06/18/2019
• 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): Ahn, Sejoon
• Application Number:
  US14/788,532
• Publication Number:
  US 20170006245A1
• Time in Patent Office: 1,449 Days
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive 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 including a sense element. Each of the sensing pixels also includes 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 non-linear circuit element, wherein the non-linear circuit element includes a first terminal connected to the sense element and the second terminal of the first transistor, and wherein the non-linear circuit element 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 configured to store charge proportional to a feature of the input object;

a first transistor comprising;

a gate terminal connected to a row select line; and

a second terminal connected to the sense element, wherein the first transistor is configured to;

couple the sense element with an enable line in response to a high assertion of the row select line; and

isolate the sense element from the row select line in response to a low assertion of the row select line; and

a non-linear circuit element, wherein the non-linear circuit element comprises;

a first non-linear circuit element terminal connected to the sense element and the second terminal of the first transistor; and

a second non-linear circuit element terminal connected to a column output line, wherein the non-linear circuit element is configured to provide a steady state current to the column output line corresponding to characteristics of the non-linear circuit element,

wherein the stored charge on the sense element is transferred through the non-linear circuit element to a feedback capacitor, and a reset switch is closed and the row select line is asserted high after reading an output voltage, wherein the output voltage is proportional to the feature of the input object.

2. The input device of claim 1, further comprising:

a reference capacitor connected to the first non-linear circuit element terminal and the second non-linear circuit element terminal of the non-linear circuit element.

3. The input device of claim 1, further comprising:

a processing system, comprising;

a charge integrating amplifier circuit connected to the column output line and configured to output the output voltage.

4. The input device of claim 1, wherein the non-linear circuit element is a diode-connected transistor.

5. The input device of claim 1, wherein the non-linear circuit element is a diode.

6. The input device of claim 3, wherein the charge integrating amplifier circuit further comprises the feedback capacitor and the reset switch.

7. The input device of claim 1, wherein the first transistor is a thin-film transistor.

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. The input device of claim 1, wherein the first transistor comprises a third terminal connected to the enable line.

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

a readout circuit, wherein the readout circuit comprises a charge integrating amplifier circuit connected to a column output line and configured to;

output a voltage representing the input object; and

measure a current corresponding to characteristics of a non-linear circuit element;

wherein driver circuitry is configured to;

connect a sense element to an enable line through a first transistor by asserting a row select line high;

isolate the sense element from the enable line by asserting the row select line;

transfer a charge stored on the sense element through the non-linear circuit element to a feedback capacitor of the charge integrating amplifier circuit; and

after readout of the voltage, assert the row select signal high and close a reset switch of the charge integrating amplifier circuit.

11. The processing system of claim 10, wherein the voltage representing the input object corresponds to a capacitance between the sense element and the input object.

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

13. The processing system of claim 10, wherein the non-linear circuit element is a diode-connected transistor.

14. The processing system of claim 10, wherein the non-linear circuit element is a diode.

15. The processing system of claim 10, wherein the reset switch is configured to reset the charge stored on the feedback capacitor after the readout of the voltage representing the input object.

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

asserting a row select line high to couple a sense element to an enable line through a first transistor, wherein the row select line is coupled to a gate terminal of the first transistor, and wherein a second terminal of the first transistor is coupled to the sense element;

collecting a charge at the sense element, wherein the charge is proportional to a feature of an input object;

asserting the row select line and the enable line low to isolate the sense element from the enable line and transfer the charge stored on the sense element to a feedback capacitor through a non-linear circuit element;

reading an output voltage, wherein the output voltage is proportional to the feature of the input object; and

wherein the non-linear circuit element is configured to provide a steady state current corresponding to characteristics of the non-linear circuit element; and

after reading the output voltage, asserting the row select line high and closing a reset switch.

17. The method of claim 16, wherein the output voltage corresponds to a capacitance between the sense element and the input object.

18. The method of claim 16, wherein the sense element has a pitch suitable for distinguishing between features of a fingerprint.

19. The method of claim 16, further comprising:

connecting the row select line to a low voltage to prepare the sense element to collect charge on another sense plate in the same column.