Abstract: A combination of a semiconductor region essentially consisting of Al.sub.x Ga.sub.1-x As (0.ltoreq.x.ltoreq.1), an insulating film formed on the surface of the semiconductor region and essentially consisting of GaAs.sub.x P.sub.y O.sub.z (w, y, z>0), and a passivation film formed on the insulating film and made of an insulating material different from the insulating film. The laminated insulating film has an extremely low leakage current. An excellent MISFET can be realized by forming a gate electrode on the surface of the laminated insulating film.