Active pixel sensor with nanowire structured photodetectors
View Patent ↗An imaging device formed as an active pixel array combining a CMOS fabrication process and a nanowire fabrication process. The pixels in the array may include a single or multiple photogates surrounding the nanowire. The photogates control the potential profile in the nanowire, allowing accumulation of photo-generated charges in the nanowire and transfer of the charges for signal readout. Each pixel may include a readout circuit which may include a reset transistor, charge transfer switch transistor, source follower amplifier, and pixel select transistor. A nanowire is generally structured as a vertical rod on the bulk semiconductor substrate to receive light energy impinging onto the tip of the nanowire. The nanowire may be configured to function as either a photodetector or a waveguide configured to guild the light to the substrate. Light of different wavelengths can be detected using the imaging device.
1. A device comprising:
a substrate having a front side and a back side opposite the front side;
a nanowire photodetector located on the front side; and
an active pixel readout circuit within the substrate in the back side.
2. The device of claim 1 , wherein the nanowire photodetector comprises a photo conductor, a photodiode, or a photogate.
3. The device of claim 1 , further comprising at least one vertical photogate surrounding the nanowire photodetector.
4. The device of claim 3 , wherein the vertical photogate is configured to electrically passivate the surface of the nanowire, suppressing dark current.
5. The device of claim 4 , wherein photo carrier lifetime is increase relative to a non-passivated nanowire photodiode.
6. The device of claim 4 , wherein the device has greater quantum efficiency relative to a non-passivated nanowire photodiode.
7. The device of claim 3 , wherein the nanowire comprises an n-type semiconductor.
8. The device of claim 7 , wherein a negative bias applied to the nanowire causes depletion of charge carriers in the nanowire.
9. The device of claim 3 , wherein the nanowire comprises a p-type semiconductor.
10. The device of claim 9 , wherein a positive bias applied to the nanowire causes depletion of charge carriers in the nanowire.
11. The device of claim 1 , wherein the active pixel readout circuit comprises three transistors in a 3-T configuration.
12. The device of claim 1 , wherein the active pixel readout circuit comprises four transistors in a 4-T configuration.
13. The device of claim 3 , further comprising a microlens coupler operatively attached to the nanowire.
14. The device of claim 13 , wherein the microlens coupler is a spherical ball lens or a binary microlens.
15. The device of claim 8 , wherein an increase of the negative bias above a threshold inverts the surface of the nanowire.
16. The device of claim 10 , wherein increase of the negative bias above a threshold depletes the surface and the core of the nanowire of mobile charges.
17. The device of claim 3 , further comprising a dielectric cladding layer between the photogate and the nanowire.
18. The device of claim 17 , wherein the thickness of the dielectric cladding layer varies along the nanowire in an axial direction of the nanowire.
19. The device of claim 3 , further comprises a substrate photodiode.
20. The device of claim 19 , wherein the substrate photodiode is located on the back side.
21. The device of claim 19 , wherein both the nanowire and the substrate photodiode are located on the same side of the substrate.
22. A device comprising:
an array of pixels, each pixel comprising a substrate having a front side and a back side opposite the front side, a nanowire photodetector located on the front side, an active pixel readout circuit within the substrate in the back side.
23. The device of claim 22 , wherein the device comprises a monolithic CMOS circuit.
24. The device of claim 22 , wherein the axial direction of the nanowire is substantially perpendicular to the substrate.
25. The device of claim 22 , wherein the presence of two photodetectors allows the collection of electromagnetic radiations of different wavelengths.
26. A device comprising:
a substrate;
a nanowire photodetector located on the substrate, the nanowire photodetector comprising a nanowire having a surface and a core, wherein the axial direction of the nanowire is at an angle to the substrate; and
an active pixel readout circuit within the substrate; wherein the axial direction of the nanowire is substantially perpendicular to the substrate; wherein the nanowire photodetector is capable of detecting light.
27. The device of claim 26 , wherein the nanowire comprises GaAs.