Pixel circuit having capacitor with identical electrodes to improve sensing efficiency and operating method thereof

1. A sensor circuit, comprising:

a photodiode;

a first voltage source having a first reset voltage;

a second voltage source having a second reset voltage;

a capacitor having a first electrode and a second electrode, the first electrode coupled to a floating diffusion node and having a sensing voltage, generated by the photodiode within an exposure interval, at a start time of a storage interval, and the second electrode having the second reset voltage at the start time of the storage interval, wherein the first and second electrodes have an identical physical characteristic;

a sustain transistor connected between the second electrode of the capacitor and the second voltage source; and

a readout circuit configured to

read, after the storage interval and when the sustain transistor is conducted, a first readout voltage on the first electrode of the capacitor,

read, after the first readout voltage is read and the first electrode of the capacitor is connected to the first voltage source, a second readout voltage on the first electrode of the capacitor, and

calculate a voltage difference between the first readout voltage and the second readout voltage to cancel out a noise voltage generated within the storage interval.

2. The sensor circuit as claimed in claim 1 , wherein within the exposure interval, the sustain transistor is conducted to maintain the second electrode of the capacitor to the second reset voltage.

3. The sensor circuit as claimed in claim 1 , wherein the first and second electrodes have identical areas, shapes and doping concentration.

4. The sensor circuit as claimed in claim 1 , further comprising:

a reset transistor, coupled between the photodiode and the first voltage source; and

a transfer control transistor, coupled between the photodiode and the floating diffusion node.

5. The sensor circuit as claimed in claim 4 , wherein

the reset transistor and the transfer control transistor are turned on within the exposure interval, and

the reset transistor and the transfer control transistor are turned off within the storage interval.

6. The sensor circuit as claimed in claim 4 , further comprising:

a first source follower transistor coupled between the photodiode and the transfer control transistor; and

a second source follower transistor coupled between the floating diffusion node and the readout circuit.

7. An operating method of a sensor circuit, the sensor circuit comprising a photodiode, a reset transistor coupled between the photodiode and a first reset voltage, a capacitor having a first electrode, which is connected to a floating diffusion node, and a second electrode identical to the first electrode, a transfer control transistor coupled between the photodiode and the floating diffusion node, as well as a sustain transistor coupled between the second electrode and a second reset voltage, the operating method comprising:

resetting the first electrode to the first reset voltage and resetting the second electrode to the second reset voltage by conducting the reset transistor, the transfer control transistor and the sustain transistor;

exposing the photodiode to store a sensing voltage on the floating diffusion node by turning off the reset transistor;

storing the sensing voltage on the floating diffusion node in a storage interval within which the reset transistor, the transfer control transistor and the sustain transistor are turned off;

reading the sensing voltage from the floating diffusion node after the sustain transistor is turned on but the reset transistor and the transfer control signal are stilled turned off;

resetting the first electrode and the second electrode of the capacitor again by conducting the reset transistor, the transfer control transistor and the sustain transistor; and

reading the first reset voltage from the floating diffusion node after the reset transistor and the transfer control signal are turned off but the sustain transistor is still conducted.

8. The operating method as claimed in claim 7 , wherein the first electrode and second electrode have identical areas, shapes and doping concentration.

9. The operating method as claimed in claim 7 , further comprising:

calculating a voltage difference between the read sensing voltage and the read first reset voltage.

10. A sensor circuit, comprising:

a photodiode;

a capacitor having a first electrode and a second electrode, the first electrode coupled to a floating diffusion node and having a sensing voltage, generated by the photodiode, at a start time of a storage interval, and the second electrode having a second reset voltage at the start time of the storage interval, wherein the first electrode and second electrode are identical to each other to receive identical influence from sensing noises within the storage interval;

a sustain transistor connected between the second electrode of the capacitor and the second reset voltage; and

a readout circuit configured to be connected to the floating diffusion node and the first electrode of the capacitor to read a voltage on the floating diffusion node.

11. The sensor circuit as claimed in claim 10 , wherein the first electrode and second electrode have identical areas, shapes and doping concentration.

12. The sensor circuit as claimed in claim 10 , further comprising:

a reset transistor, coupled between the photodiode and a first reset voltage; and

a transfer control transistor, coupled between the photodiode and the floating diffusion node.

13. The sensor circuit as claimed in claim 12 , wherein the reset transistor, the transfer control transistor and the sustain transistor are all turned off within the storage interval.

14. The sensor circuit as claimed in claim 13 , wherein the readout circuit is configured to

read the voltage on the floating diffusion node for a first time when the sustain transistor is turned on after the storage interval, and

read the voltage on the floating diffusion node for a second time after the voltage on the floating diffusion node is reset to the first reset voltage.

15. The sensor circuit as claimed in claim 14 , wherein the readout circuit is further configured to calculate a voltage difference between the firstly read voltage and the secondly read voltage from the floating diffusion node to cancel out the influence from the sensing noises.