A planar silicon photosensor with an optimal spectral response for detecting printed material

In designing the optimum photosensing device for use in the detection of printed material in a reading aid, experimental evidence points out that the device should have a spectral response which matches that of the human eye. It is demonstrated theoretically that for a given set of base-region recombinative properties, there is an optimum base-collector junction depth for achieving the desired visible region sensitivity. In addition, by truncating the collector region with an epitaxial collector-substrate junction, the desired insensitivity to near-infrared can be achieved. For the recombinative properties typical of the experimental planar devices fabricated in the Integrated Circuits Laboratory at Stanford University, it is theoretically shown and experimentally verified that the optimum base-collector and epitaxial junction depths are nominally 2µ and 8µ, respectively. Accomplishment of this in silicon, which was one of the basic objectives of the research, facilitates monolithic integration of the optimum photosensor with the remainder of the reading aid circuitry.