The semiconductor device of the present invention includes a device formation region formed on a substrate and including at least one semiconductor region, and a first electrode and a second electrode formed spaced apart from each other on the device formation region. A semi-insulating film is formed to cover the surface of a portion of the semiconductor region, which portion is located between the first and second electrodes and in which portion a depletion layer extends when a reverse bias is applied between the first and second electrodes.