PURPOSE: To prevent deterioration in interface characteristics and to stabilize the same,by forming on a compound semiconductor substrate a region having a different conductivity type from that of the substrate, forming an electrode on said region, covering the periphery of the region with an protecting insulation film and forming an anodic sulfurated film on the surface of the substrate.
CONSTITUTION: An insulation film 2 of SiO 2 is formed on an InSb crystal substrate 1 having the N conductivity type. After applied on the surface of the insulation film, a resist film is patterned to form a resist mask pattern 21 to be used in ion implantation. A P-type region 4 is provided in the crystal substrate 1 by the ion implantation. The resist mask pattern 21 and the insulation film 2 are once removed completely. Then, an electrode 22 is provided on the P-type region and a first protecting insulation film 23 is deposited so as to cover the periphery of the electrode 22. An anodic sulfurated film 24 is formed by the anodic sulfaration process on the surface of the substrate except the first protecting insulation film 23, Then a second protecting insulation film 25 is formed of SiO 2 , the same material with the first protecting insulation film 23, so as to cover the first protecting insulation film 23 and the anodic sulfurated film 25 and an aperture 26 is formed therein. In this manner, the interface between the substrate 1 and the first protecting insulation film 23 is protected against pollution and, therefore, allowed to have improved electric characteristics.