Reducing reset noise in CMOS image sensors

1. A CMOS image sensor comprising:

a reset transistor having a drain, a gate and a source;

a source follower transistor having a drain, a gate and a source;

a photodiode having a cathode connected to said source of said reset transistor and said gate of said source follower transistor, said photodiode having an anode connected to ground;

reset drain voltage circuitry connected to said drain of said reset transistor and said drain of said source follower transistor;

reset gate voltage circuitry connected to said gate of said reset transistor, wherein during a hard reset operation involving said photodiode, said reset drain voltage circuitry supplies a first drain voltage to said drain of said reset transistor in accordance with a determined level of light for exposure, and said reset gate voltage circuitry supplies a first gate voltage to said gate of said reset transistor such that said first gate voltage is greater than a sum of said first drain voltage and a threshold voltage of said reset transistor.

2. The CMOS image sensor of claim 1 , wherein said level of light for exposure is determined dynamically.

3. The CMOS image sensor of claim 1 , wherein said hard reset operation is carried out over either one or two clock cycles.

4. The CMOS image sensor of claim 1 , wherein during said hard reset operation, said first gate voltage is approximately 3.3 volts, and said first drain voltage is either approximately 2.3 volts for bright conditions or approximately 1.5 volts for low light conditions.

5. A CMOS image sensor comprising:

a reset transistor having a drain, a gate and a source;

a photodiode having a cathode connected to said source of said reset transistor, said photodiode having an anode connected to ground;

reset drain voltage circuitry connected to said drain of said reset transistor;

reset gate voltage circuitry connected to said gate of said reset transistor, wherein during a hard reset operation involving said photodiode, said reset drain voltage circuitry supplies a first drain voltage to said drain of said reset transistor in accordance with a determined level of light for exposure, and said reset gate voltage circuitry supplies a first gate voltage to said gate of said reset transistor such that said first gate voltage is greater than a sum of said first drain voltage and a threshold voltage of said reset transistor;

wherein said hard reset operation is immediately followed by a soft reset operation, wherein during said soft reset operation, said reset drain voltage circuitry supplies a second drain voltage to said drain approximately equivalent to a second gate voltage supplied by said reset gate voltage circuitry to said gate so that said reset transistor performs sub-threshold conduction.

6. The CMOS image sensor of claim 5 , wherein said second gate voltage supplied by said reset gate voltage circuitry is held at a constant level during said soft reset operation.

7. The CMOS image sensor of claim 5 , wherein said second gate voltage and said second drain voltage is approximately 3.3 volts during said soft reset operation.

8. A CMOS image sensor comprising:

a reset transistor having a drain, a gate and a source;

a photodiode having a cathode connected to said source of said reset transistor, said photodiode having an anode connected to ground;

reset drain voltage circuitry connected to said drain of said reset transistor;

reset gate voltage circuitry connected to said gate of said reset transistor, wherein during a hard reset operation involving said photodiode, said reset drain voltage circuitry supplies a first drain voltage to said drain of said reset transistor in accordance with a determined level of light for exposure, and said reset gate voltage circuitry supplies a first gate voltage to said gate of said reset transistor such that said first gate voltage is greater than a sum of said first drain voltage and a threshold voltage of said reset transistor;

wherein said hard reset operation is immediately followed by a soft reset operation, wherein during said soft reset operation, said reset drain voltage circuitry supplies a second drain voltage to said drain approximately at a maximum potential, and reset gate voltage circuitry supplies a second gate voltage to said gate which ramps down so that said reset transistor performs subthreshold conduction.

9. The CMOS image sensor of claim 8 , wherein said second drain voltage is approximately 3.3 volts, and said second gate voltage ramps from approximately 4.8 volts to approximately 2.7 volts during said soft reset operation.

10. A CMOS image sensor comprising:

a reset transistor having a drain, a gate and a source;

a photodiode having a cathode connected to said source of said reset transistor, said photodiode having an anode connected to ground;

reset drain voltage circuitry connected to said drain of said reset transistor;

reset gate voltage circuitry connected to said gate of said reset transistor, wherein during a first phase of reset operation involving said photodiode, said reset drain voltage circuitry supplies a first drain voltage to said drain of said reset transistor, and said reset gate voltage circuitry supplies a first gate voltage to said gate of said reset transistor such that said first gate voltage is greater than a sum of said first drain voltage and a threshold voltage of said reset transistor, and during a second phase of said reset operation, said reset drain voltage circuitry supplies a second drain voltage to said drain, and reset gate voltage circuitry supplies a second gate voltage to said gate so that said reset transistor performs sub-threshold conduction.

11. The CMOS image sensor of claim of claim 10 , wherein second drain voltage is approximately equivalent to said second gate voltage supplied by said reset gate voltage circuitry.

12. The CMOS image sensor of claim 11 , wherein said second gate voltage supplied by said reset gate voltage circuitry is held at a constant level during said second phase.

13. The CMOS image sensor of claim 11 , wherein said second gate voltage and said second drain voltage is approximately 3.3 volts during said second phase.

14. The CMOS image sensor of claim 10 , wherein said second gate voltage ramps down from a higher potential to a lower potential during said second phase.

15. The CMOS image sensor of claim 14 , wherein said second drain voltage is approximately 3.3 volts, and said second gate voltage ramps down from approximately 4.8 volts to approximately 2.7 volts during said second phase.

16. The CMOS image sensor of claim 14 , wherein said first drain voltage is supplied in accordance with a determined level of light for exposure.

17. A method of resetting a photodiode in a CMOS image sensor, said photodiode having a cathode connected to a source of a reset transistor, said photodiode having an anode connected to ground, said method comprising the steps of:

determining a level of light for exposure;

supplying a first drain voltage to a drain of said reset transistor in accordance with said determined level of light for exposure;

supplying a first gate voltage to a gate of said reset transistor such that said first gate voltage is greater than a sum of said first drain voltage and a threshold voltage of said reset transistor;

supplying a second gate voltage supplied to said gate; and

supplying a second drain voltage to said drain so that said reset transistor performs sub-threshold conduction.

18. A method of resetting a photodiode in a CMOS image sensor, said photodiode having a cathode connected to a source of a reset transistor, said photodiode having an anode connected to ground, said method comprising the steps of:

supplying a first drain voltage to a drain of said reset transistor during a first phase;

supplying a first gate voltage to a gate of said reset transistor such that said first gate voltage is greater than a sum of said first drain voltage and a threshold voltage of said reset transistor during said first phase;

supplying a second gate voltage supplied to said gate during a second phase;

supplying a second drain voltage to said drain so that said reset transistor performs sub-threshold conduction during said second phase.

19. The method of claim 18 , wherein second drain voltage is approximately equivalent to said second gate voltage during said second phase.

20. The method of claim 18 , wherein said second gate voltage ramps down from a higher potential to a lower potential during said second phase.

21. The method of claim 18 , wherein said first drain voltage is supplied in accordance with a determined level of light for exposure.

22. A method of resetting a photodiode in a CMOS image sensor, said photodiode having a cathode connected to a source of a reset transistor and a gate of a source follower transistor, said photodiode having an anode connected to ground, said method comprising the steps of:

determining a level of light for exposure;

supplying a first drain voltage to a drain of said reset transistor and a drain of said source follower transistor in accordance with said determined level of light for exposure;

supplying a first gate voltage to a gate of said reset transistor such that said first gate voltage is greater than a sum of said first drain voltage and a threshold voltage of said reset transistor.