Patent ID: 7776638

Claim:
A method for making an image sensor, the method comprising: forming a first epitaxial layer of a first conductivity type on a substrate of the first conductivity type; forming a subcollector of a second conductivity type in the first epitaxial layer, wherein the second conductivity type is opposite the first conductivity type; forming one or more subcollector contact regions of the second conductivity type in the first epitaxial layer, wherein the one or more subcollector contact regions connect to the subcollector; forming a second epitaxial layer of the first conductivity type on the first epitaxial layer; when forming the second epitaxial layer on the first epitaxial layer, diffusing one or more dopants in the subcollector and in the one or more subcollector contact regions, wherein the dopants in the one or more subcollector contact regions diffuse at a faster rate than the one or more dopants in the subcollector so that a portion of each subcollector contact region diffuses into the second epitaxial layer; and forming contact regions of the second conductivity type in the second epitaxial laver, wherein each contact region connects to the portion of a respective subcollector contact region that diffused into the second epitaxial layer.