Imaging pixels with non-destructive readout capabilities
An image sensor may include imaging pixels with non-destructive readout capabilities. Each imaging pixel may include a substrate having a photosensitive area that generates charge in response to incident light. The charge may accumulate at a front side of the substrate adjacent to a floating gate. The voltage of the floating gate may depend on how much charge is accumulated. The voltage of the floating gate may be repeatedly sampled to monitor the amount of incident light received over time. A first reset transistor may clear the substrate of accumulated charge. A second reset transistor may reset the voltage of the floating gate. The imaging pixel may be split between multiple wafers if desired and may include nMOS and pMOS transistors.
1. An image sensor comprising:
a semiconductor substrate having first and second opposing surfaces;
a photosensitive area in the semiconductor substrate, wherein the photosensitive area is configured to generate charge in response to incident light during an integration period;
a floating gate formed adjacent to the first surface of the semiconductor substrate, wherein the floating gate and the photosensitive area overlap and wherein the charge generated by the photosensitive area accumulates at the first surface of the semiconductor substrate adjacent to the floating gate; and
a source follower transistor, wherein the floating gate is electrically connected to a gate of the source follower transistor.
2. The image sensor defined in claim 1 , further comprising:
readout circuitry configured to sample a voltage of the floating gate multiple times within the integration period.
3. The image sensor defined in claim 2 , wherein the readout circuitry is further configured to sample a reset voltage of the floating gate after sampling the voltage of the floating gate multiple times within the integration period.
4. The image sensor defined in claim 1 , further comprising:
gate oxide formed on the first surface, wherein the floating gate is formed on the gate oxide.
5. The image sensor defined in claim 1 , further comprising:
a reset transistor formed adjacent to the floating gate adjacent to the first surface of the semiconductor substrate, wherein the reset transistor has a gate, wherein the gate and the photosensitive area overlap, and wherein the gate and the floating gate are coplanar.
6. The image sensor defined in claim 5 , wherein the reset transistor is configured to clear the photosensitive area of accumulated charge.
7. The image sensor defined in claim 5 , wherein the reset transistor is an n-channel metal-oxide-semiconductor transistor and the source follower transistor is a p-channel metal-oxide-semiconductor transistor.
8. The image sensor defined in claim 7 , wherein the source follower transistor is formed in an additional semiconductor substrate that is connected to the first semiconductor substrate with a conductive interconnect layer.
9. The image sensor defined in claim 1 , further comprising:
a reset transistor coupled between the floating gate and a bias voltage supply terminal, wherein the reset transistor is configured to reset a voltage of the floating gate.
10. An image sensor comprising:
a substrate having first and second opposing sides;
a photosensitive area in the substrate that is configured to generate charge in response to incident light during an integration time;
a floating gate on the first side of the substrate, wherein the charge generated by the photosensitive area accumulates in the substrate adjacent to the floating gate;
a source follower transistor, wherein the floating gate is electrically connected to a gate of the source follower transistor;
a reset transistor formed adjacent to the floating gate on the first side of the substrate; and
readout circuitry configured to sample a voltage of the floating gate multiple times during the integration time, wherein the reset transistor is an n-channel metal-oxide-semiconductor transistor and the source follower transistor is a p-channel metal-oxide-semiconductor transistor.
11. The image sensor defined in claim 10 , wherein the voltage of the floating gate is dependent upon an amount of charge accumulated in the substrate adjacent to the floating gate.
12. The image sensor defined in claim 10 , wherein the reset transistor is a first reset transistor that is configured to clear the charge accumulated in the substrate, the image sensor further comprising:
a second reset transistor configured to reset the voltage of the floating gate.
13. The image sensor defined in claim 12 , wherein the first reset transistor has a gate that is formed adjacent the floating gate on the first side of the substrate and wherein the gate of the first reset transistor and the floating gate are formed over a buried channel on the first side of the substrate.
14. The image sensor defined in claim 13 , wherein the second reset transistor is coupled between the floating gate and a bias voltage supply terminal.
15. An image sensor comprising:
a first substrate;
a photosensitive area formed in the first substrate;
a floating gate adjacent to the photosensitive area in the first substrate;
a second substrate;
a source follower transistor in the second substrate; and
a metal interconnect layer between the first and second substrates that electrically connects the floating gate in the first substrate to a gate of the source follower transistor in the second substrate.
16. The image sensor defined in claim 15 , further comprising:
a first reset transistor in the first substrate configured to clear charge from the photosensitive area; and
a second reset transistor in the second substrate configured to reset a voltage of the floating gate.
17. The image sensor defined in claim 16 , wherein the first reset transistor is an n-channel metal-oxide-semiconductor transistor, wherein the second reset transistor is a p-channel metal-oxide-semiconductor transistor, and wherein the source follower transistor is a p-channel metal-oxide-semiconductor transistor.
18. The image sensor defined in claim 17 , further comprising:
a capacitor in the second substrate;
a sampling transistor coupled between the source follower transistor and the capacitor.
19. The image sensor defined in claim 18 , further comprising:
a first row select transistor coupled to the source follower transistor, wherein the sampling transistor is a p-channel metal-oxide-semiconductor transistor and the first row select transistor is a p-channel metal-oxide-semiconductor transistor;
an additional source follower transistor, wherein the capacitor is coupled between the sampling transistor and the additional source follower transistor and wherein the additional source follower transistor is a p-channel metal-oxide-semiconductor transistor; and
a second row select transistor coupled to the additional source follower transistor, wherein the second row select transistor is a p-channel metal-oxide-semiconductor transistor.
20. The image sensor defined in claim 15 , further comprising:
readout circuitry configured to sample a voltage of the floating gate multiple times within an integration period.
21. The image sensor defined in claim 1 , wherein the photosensitive area comprises a photodiode having a potential gradient that sweeps the charge generated by the photodiode to the first surface of the semiconductor substrate adjacent to the floating gate.