PURPOSE: To avoid the deterioration in breakdown strength and the breakdown of a gate oxide film by a method wherein a process to form the second conductive film is performed on the whole surface of a semiconductor substrate so that the potential difference may not be made between a gate electrode and the substrate in ion-implantation process.

CONSTITUTION: A gate oxide film 2 is formed on the main surface of a silicon substrate 1 by thermal oxidation. Next, the first polycrystalline silicon 3 to be a gate electrode is formed on the oxide film 2 by CVD process. Next, the silicon 3 and the oxide film 2 are patterned by dryetching process. Next, the second polycrystalline silicon thin film 5 is formed on the main surface, side surfaces and rear surface of the substrate 1. n+ type diffused regions 4a, 4b to be a source or a drain are formed by implanting ions of an impurity such as As+, etc., and then the thin film 5 is converted into a silicon oxide 6 as an insulator. Resultantly, the potential difference will not be made between the silicon substrate and the polycrystalline silicon as a gate electrode in the ion-implantation process.