SEMICONDUCTOR DEVICE

PURPOSE:To improve writing efficiency and retaining characteristics, by providing a specific value or more of the concentration of impurities, which are added into polycrystal silicon constituting a floating gate. CONSTITUTION:An oxide film 2 is formed on a silicon substrate 1, and thereafter a first silicon oxide film 3 is formed by heat treatment. Then a first polycrystal silicon film 7 is grown on said silicon film 3. Thereafter, phosphorus is added into the silicon film at a concentration of 1X10 cm. Then the silicon films 3 and 7 are selectively removed, and a second silicon film 5 is formed on the substrate exposed by the removal. A silicon film 6 is formed, and phosphorus is diffused in the silicon film 6 at a concentration of e.g., about 2X10 cm. Thereafter, the silicon films 3 and 5-7 are selectively removed in a self-aligning way, and As is implanted. Thus source and drain regions are formed.