PURPOSE: To realize an EPROM with excellent readability by a method wherein ions of a p-type impurity with a high diffusion efficiency are implanted for the formation of a impurity layer, a little deep in a channel region, with its impurity concentration suitable for the generation of hot electrons.

CONSTITUTION: An element-isolating oxide film 12 and gate oxide film 13 are formed on a p-type Si substrate 11. Next, boron ions 17 are implanted into a channel region, when impurity concentration is carefully adjusted. A process follows wherein arsenic ions 18 are implanted into a memory cell region only. In this process, implantation is so accomplished that the center of implantation be near the surface of the Si substrate 11 and that the quantity of implantation be not more than the boron concentration after heat treatment. Accordingly, impurity in the channel region in the Si substrate 11 shows the maximum in its concentration at a level a little deep from the surface and said maximum value is quite suitable for the generation of hot electrons.