Abstract: A semiconductor photodetector includes a semiconductor layer disposed on a main surface of a first conductivity type semiconductor substrate and a metal layer producing a Schottky junction with the semiconductor layer disposed on a main surface of the semiconductor layer. The semiconductor layer has such an energy band gap in the neighborhood of the Schottky junction in a depletion layer in the first conductivity type semiconductor substrate and the semiconductor layer that no barrier to the flow of majority carriers from the metal layer to the first conductivity type semiconductor substrate is produced and a barrier to the flow of minority carriers from the first conductivity type semiconductor substrate to the metal layer is produced. Therefore, a semiconductor photodetector that suppresses the dark current caused by thermal excitation and noise in the detected signal is obtained.