Image sensors having high dynamic range pixels
An image sensor may include an array of imaging pixels arranged in rows and columns. Each imaging pixel may include a photodiode, an overflow capacitor, an overflow transistor that is interposed between the photodiode and the overflow capacitor, a floating diffusion region, a transfer transistor that is interposed between the photodiode and the floating diffusion region, a voltage supply, and a reset transistor that is interposed between the floating diffusion region and the voltage supply. The voltage supply may provide a voltage at a first magnitude that is less than the pinning voltage for a first portion of a reset period and may provide the voltage at a second magnitude that is greater than the pinning voltage for a second portion of the reset period.
1. A method of operating an imaging pixel that comprises a photodiode, a floating diffusion region, a first transistor coupled between the photodiode and the floating diffusion region, a capacitor, a second transistor coupled between the photodiode and the capacitor, a voltage supply, and a third transistor coupled between the floating diffusion region and the voltage supply, the method comprising:
during a first portion of a reset period, asserting the first, second, and third transistors while the voltage supply provides a voltage at a first magnitude that is less than a pinning voltage of the photodiode; and
during a second portion of a reset period, asserting the first and third transistors while the voltage supply provides the voltage at a second magnitude that is greater than the pinning voltage of the photodiode.
2. The method defined in claim 1 , further comprising:
during an integration period, storing charge at the photodiode and the capacitor.
3. The method defined in claim 2 , further comprising:
during the integration period, pulsing the second transistor to an intermediate level.
4. The method defined in claim 3 , further comprising:
during a readout period that is subsequent to the integration period, sampling a first reset level.
5. The method defined in claim 4 , further comprising, during the readout period:
asserting the first transistor to transfer charge from the photodiode to the floating diffusion region; and
sampling a first signal level.
6. The method defined in claim 5 , further comprising, during the readout period and after asserting the first transistor to transfer charge from the photodiode to the floating diffusion region:
asserting the first transistor and the second transistor; and
sampling a second signal level.
7. The method defined in claim 6 , further comprising, during the readout period and after sampling the second signal level:
asserting the first, second, and third transistors while the voltage supply provides the voltage at the first magnitude;
asserting the first, second, and third transistors while the voltage supply provides the voltage at the second magnitude; and
sampling a second reset level.
8. The method defined in claim 1 , wherein the imaging pixel comprises an additional voltage supply and wherein the capacitor is coupled between the additional voltage supply and the second transistor.
9. The method defined in claim 8 , further comprising:
during the reset period and using the additional voltage supply, providing an additional voltage at a third magnitude.
10. The method defined in claim 9 , further comprising:
during the integration period and using the additional voltage supply, providing the additional voltage at a fourth magnitude that is less than the third magnitude.
11. The method defined in claim 10 , further comprising:
during a readout period and using the additional voltage supply, providing the additional voltage at a fifth magnitude that is less than the third magnitude.
12. The method defined in claim 11 , wherein the fifth magnitude is greater than the fourth magnitude.
13. The method defined in claim 1 , further comprising:
asserting the second transistor during the second portion of the reset period.
14. An imaging pixel, comprising:
a photodiode that has a pinning voltage;
a floating diffusion region;
a first transistor coupled between the photodiode and the floating diffusion region;
a capacitor;
a second transistor coupled between the photodiode and the capacitor;
a voltage supply configured to provide a voltage; and
a third transistor coupled between the floating diffusion region and the voltage supply, wherein the voltage supply is configured to, during a first portion of a reset period, provide the voltage at a first magnitude that is less than the pinning voltage and wherein the voltage supply is configured to, during a second portion of the reset period, provide the voltage at a second magnitude that is greater than the pinning voltage.
15. The imaging pixel defined in claim 14 , further comprising:
an additional photodiode that has an additional pinning voltage;
a fourth transistor coupled between the additional photodiode and the floating diffusion region;
an additional capacitor; and
a fifth transistor coupled between the additional photodiode and the additional capacitor, wherein the first magnitude is less than the additional pinning voltage and wherein the second magnitude is greater than the additional pinning voltage.
16. The imaging pixel defined in claim 14 , further comprising:
an additional capacitor;
a fourth transistor coupled between the additional capacitor and the capacitor, wherein the additional capacitor is connected to the photodiode in series with the capacitor.
17. The imaging pixel defined in claim 14 , further comprising:
an additional capacitor;
a fourth transistor coupled between the additional capacitor and the photodiode, wherein the additional capacitor is connected to the photodiode in parallel with the capacitor.
18. The imaging pixel defined in claim 14 , further comprising:
an additional capacitor;
a fourth transistor coupled between the additional capacitor and the floating diffusion region.
19. The imaging pixel defined in claim 14 , further comprising:
a first semiconductor substrate that includes the photodiode, the first transistor, the capacitor, and the second transistor;
a second semiconductor substrate that includes the floating diffusion region, the voltage supply, and the third transistor; and
a conductive interconnect that electrically connects the first transistor in the first semiconductor substrate to the floating diffusion region in the second semiconductor substrate.
20. A method of operating an imaging pixel that comprises a photodiode, a floating diffusion region, a first transistor coupled between the photodiode and the floating diffusion region, a capacitor, a second transistor coupled between the photodiode and the capacitor, and a voltage supply, wherein the capacitor is coupled between the voltage supply and the second transistor, wherein the first and second transistors are connected in series between the capacitor and the floating diffusion region, and wherein the method comprises:
during a reset period and using the voltage supply, providing a voltage at a first magnitude;
during an integration period, storing charge at the photodiode and at the capacitor; and
during a readout period and using the voltage supply, providing the voltage at a second magnitude that is less than the first magnitude.
21. The method defined in claim 20 , wherein the photodiode is directly connected to a node and wherein the node is interposed between the first and second transistors.
22. The method defined in claim 20 , wherein the capacitor has a first plate coupled directly to the voltage supply and wherein the capacitor has a second plate coupled directly to the second transistor.