Patent ID: 8039287

Claim:
A method of forming a pixel cell for an image sensor, the method comprising: forming a photodiode, the act of forming the photodiode comprising forming a plurality of layers within a substrate, the plurality of layers comprising at least a first layer of a first material, a second layer of a second material, a third layer of the first material, and a fourth layer of the second material, wherein the first material has a first band gap and the second material has a second band gap, wherein layers of like material are not in contact with one another, wherein the first, second, third and fourth layers are configured to promote ionization by a first carrier type and suppress ionization by a second carrier type in the presence of an electric field, wherein the act of forming the layers comprises doping a first subset of layers to a first conductivity type and doping at least a second subset of layers to a second conductivity type, and wherein the first conductivity type is different than the second conductivity type; forming a buffer between a bottom layer of the photodiode and the substrate, the act of forming the buffer layer comprises forming a buffer layer with a composition that changes from a bottom surface of the buffer layer to a top surface of the buffer layer, wherein the bottom layer of the photodiode and the top surface of the buffer layer have approximately the same chemical composition; forming a first transistor for transferring the first carrier type from the photodiode, the transistor formed adjacent to the photodiode; and forming a second transistor for coupling the pixel cell to an output line.