e, of silicon (Si) on the ohmic contact 11, forming an insulator layer 13 made, for example, of SiOx (X=0.1 to 2.0) or the like on the electron-supply layer 12, and forming a thin-film metal electrode 15 made of metal such as platinum (Pt), gold (Au) or the like, and facing a vacuum space on the insulator layer 13.