Photoconductive Gain in a Schottky Barrier Photodiode

Quantum efficiency in excess of 100% was measured for a Ni-Si-Ni Schottky-barrier photodiode. In addition, the RF impedance of the photodiode displayed strong reciprocal dependance on the optical power illuminating the photodiode, and proportional dependance on the applied bias. It is proposed here that the mechanism by which this anomalous gain occurs is the modification of the Schottky barrier by optically-generated charge; specifically, residual holes that are slower in transit than electrons. The electric field between these holes, and the corresponding image charge in the metal contact, is sufficiently strong to alter the barrier. The result of a theoretical calculation indicates that the resistance of the Schottky barrier, and consequently the photodiode, varies inversely as the square-root of the electric field. The equivalent circuit of a semiconductor photodetector is generalized to include this effect.