SOI SUBSTRATE

PURPOSE:To largely reduce an effective parasitic capacity by a method wherein an impurity, which is used as a lifetime killer to semiconductor carriers, is introduced at a position deeper than that of an insulator thin film. CONSTITUTION:An insulator thin film 3 is buried in the interior of a semiconductor substrate 1 to form a SOI semiconductor on insulator substrate. At that time, an impurity, which is used as a lifetime killer to semiconductor carriers, is introduced at a position deeper than that of the film 3. Whereupon, as a layer 6 introduced the impurity is brought in a semi-insulative state and is depleted, a capacity which is formed by the thin film 3 and the capacity of the depleted layer take a shape that both capacities are connected in series as seen from the semiconductor film of the surface layer. Thereby, an effective parasitic capacity is largely reduced.