Image sensors with noise reduction
In various embodiments, image sensors and methods of operating the image sensors are disclosed. In an example embodiment, a pixel circuit having a first electrode is coupled to a reset transistor and to a first region of an optically sensitive layer, and a second electrode is coupled to a pixel sense node and to a second region of an optically sensitive layer. The electrical path from the first electrode, through the optically sensitive layer, and into the second electrode functions as a variable resistor. Other devices and methods of operating the devices are disclosed.
1. A pixel circuit, comprising:
a first electrode coupled to a reset transistor and to a first region of an optically sensitive layer;
a second electrode coupled to a pixel sense node and to a second region of the optically sensitive layer, the electrical path from the first electrode, through the optically sensitive layer, and into the second electrode, being configured to function as a variable resistor,
wherein, in a first mode, the variable resistor exhibits a resistance of less than 10 9 ohms.
2. The pixel circuit of claim 1 , wherein, in the first mode, the sense node is reset to a potential determined by the reset transistor.
3. The pixel circuit of claim 1 , wherein, in a second mode, the variable resistor exhibits a resistance exceeding 10 16 ohms.
4. The pixel circuit of claim 3 , wherein, in the second mode, the optically sensitive layer is substantially fully depleted of free carriers.
5. The pixel circuit of claim 3 , wherein, in the second mode, the sense node potential changes at a rate related to the optical signal illuminating the optically sensitive layer.
6. The pixel circuit of claim 1 , wherein, in a third mode, the first electrode is electrically coupled to the output of a feedback amplifier.
7. The pixel circuit of claim 6 , wherein the feedback amplifier is configured to impose a voltage on the sense node that is specified to within a noise power that is appreciably less than kT/C.
8. The pixel circuit of claim 1 , wherein the optically sensitive layer is sensitive to any of one or more wavelength regions of regions including infrared, visible, and ultraviolet regions of the electromagnetic spectrum.
9. The pixel circuit of claim 1 , wherein the optically sensitive layer includes one or more types of quantum dot nanocrystals.
10. The pixel circuit of claim 9 , wherein at least two of the one or more types of quantum dot nanocrystals are at least partially fused.
11. The pixel circuit of claim 1 , wherein the optically sensitive layer includes a combination of at least two types of quantum dots, each of the at least two types of quantum dots including a distinct semiconductor material.
12. The pixel circuit of claim 1 , wherein the optically sensitive layer includes a combination of at least two types of quantum dots, each of the at least two types of quantum dots having distinct properties.
13. The pixel circuit of claim 1 , wherein the optically sensitive layer includes at least one of an optically sensitive semiconducting polymer selected from a list of polymers including MEH-PPV, P3OT, and P3HT.
14. The pixel circuit of claim 1 , wherein the optically sensitive layer includes a polymer-quantum dot mixture having one or more types of quantum dots sensitive to different portions of the electromagnetic spectrum.
15. A pixel circuit, comprising:
a first electrode coupled to a reset transistor and to a first region of an optically sensitive layer;
a second electrode coupled to a pixel sense node and to a second region of the optically sensitive layer, the electrical path from the first electrode, through the optically sensitive layer, and into the second electrode, being configured to function as a variable resistor,
wherein in a second mode, the variable resistor exhibits a resistance exceeding 10 16 ohms.
16. The pixel circuit of claim 15 , wherein, in the second mode, the optically sensitive layer is substantially fully depleted of free carriers.
17. The pixel circuit of claim 15 , wherein, in the second mode, the sense node potential changes at a rate related to the optical signal illuminating the optically sensitive layer.