PURPOSE: To prevent the change of properties of a semi-insulating GaAs substrate generated on heat treatment by depositing an insulating film, through which the substrate is not altered even through heat treatment, on the substrate and forming an electrode metal so as to be superposed on the insulating film.

CONSTITUTION: An active-region pattern is shaped onto a semi-insulating substrate 1 by a photo-resist 2. A gate 3 under the state of a Schottky-barrier contact is formed onto the substrate 1. SiO2 4 and a photo-resist 5 are shaped onto the whole surface of the substrate, and a high concentration layer is formed. An Si3N4 film 6 is shaped onto the whole surface of the substrate, an SiO2 film 7 is formed onto the film 6, and a spacer is shaped in double layers. A photo-resist 8 is applied onto the film 7, and a pattern is formed. The film 7 is etched to form overhang structure, and the film is etched in an anisotropic manner. An electrode metal 9 is evaporated, the resist 8 is lifted off, and an electrode 9' is alloyed. According to said manufacture, the evaporation of As and the oxidation of Ga are prevented on heat treatment, and the substrate 1 can be protected.