In a conventional power MOSFET, an electric field concentration occurs at a gate electrode bottom portion on the outermost periphery of an operating area, thereby causing a deterioration in high voltage strength between the drain and the source, or between the collector and emitter. A charger includes a second battery. The impurity distribution is introduced into the top layer such that the net charge deposited in the semiconductor top layer varies linearly with the thickness variation. For the solution to the problem, the silicon nitride film is oxidized by means of a high concentration ozone gas to form the top silicon oxide film. In the operating position, the bar is in contact with a stop located on a two-legged lever, one leg of which is acted upon by the beam and the other leg of which is acted upon by a energy storing mechanism.