Patent ID: 8039882

Claim:
A pixel cell for an image sensor, the pixel cell comprising: a photodiode for generating charge in response to light and for amplifying the generated charge, the photodiode being formed within a substrate and below an upper surface thereof and comprising at least two of a first layer having a first band gap and at least two of a second layer having a second band gap, wherein the first layers are alternated with the second layers, wherein the first layers are not in direct contact with one another and the second layers are not in direct contact with one another, and wherein the at least two first layers and the at least two second 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 first layers are doped to a first conductivity type, wherein the second layers are doped to a second conductivity type, and wherein the first conductivity type is different from the second conductivity type; a gate of a transistor adjacent to the photodiode, the transistor for transferring the amplified charge from the photodiode; and a graded buffer layer beneath a bottom layer of the photodiode.