Graded bandgap photodetector

The invention is a reduced noise avalanche photodetector. The detector comprises a p-type region, an n-type region, and a graded bandgap avalanche region situated between the p- and ntype regions. Radiation to be detected is absorbed in one of the p-type and n-type regions and charge carriers are generated in response thereto. When the device is under a reverse bias, one type of photogenerated charge carrier is injected by diffusion into the graded bandgap region and initiates an avalanche discharge therein. The carrier type initiating the discharge moves toward a region of decreasing bandgap energy, while the other type of charge carrier moves toward a region of increasing bandgap energy , thus resulting in a large difference between the ionization coefficients of the two types of charge carriers. The differing ''quasi-electric'' fields experienced by the two types of charge carriers also contributes to the difference between the ionization coefficients of the electrons and holes