A semiconductor device type of field effect transistor (FET) primarily made of nitride semiconductor materials is disclosed. The FET includes a nitride semiconductor stack providing primary and auxiliary active regions and an inactive region surrounding the active regions; electrodes of a source, a drain, and a gate; an insulating film covering the electrodes and the semiconductor stack; and a field plate on the insulating film. A feature of the FET of the invention is that the field plate is electrically in contact with the auxiliary active region through the opening provided in the insulating film.