Pixel with strained silicon layer for improving carrier mobility and blue response in imagers
View Patent ↗An imager having a pixel cell having an associated strained silicon layer. The strained silicon layer increases charge transfer efficiency, decreases image lag, and improves blue response in imaging devices.
1. A system comprising:
a plurality of pixel cells, wherein at least one of the pixel cells comprises:
a semiconductor substrate including a strained silicon layer at an upper portion of the semiconductor substrate, and
a photosensor for generating charge formed in an upper region of the semiconductor substrate, wherein the photosensor comprises a photosensitive region buried at least in part within the semiconductor substrate and separated from the strained silicon layer by an intermediary portion of the semiconductor substrate; and
a readout circuit formed on the semiconductor substrate and comprising at least one output transistor.
2. The system of claim 1 , wherein—
the at least one of the pixel cells further comprises a floating diffusion region; and
the at least one output transistor is a reset transistor connected to the floating diffusion region for resetting the charge on the floating diffusion region.
3. The system of claim 2 , wherein the readout circuit further comprises:
a transfer transistor connected to the photosensor and the floating diffusion region for transferring charge from the photosensor to the floating diffusion region,
a source-follower transistor having a gate for receiving charge from the floating diffusion region, and
a row select transistor connected to the source-follower transistor for outputting a signal produced by the source-follower transistor.
4. The system of claim 1 , wherein the intermediary portion comprises an undoped portion of the semiconductor substrate.
5. The system of claim 1 , wherein the photosensor is buried entirely below the strained silicon layer.
6. The system of claim 1 , wherein the strained silicon layer has a thickness of about 500 Å to about 1000 Å.
7. The system of claim 1 , wherein the strained silicon layer comprises a top silicon layer formed over a silicon-germanium layer.
8. The system of claim 7 , wherein the silicon-germanium layer is graded.
9. The system of claim 7 , wherein the silicon-germanium layer has a germanium concentration of about 30% to about 40%.