PURPOSE: To improve gate withstand voltage, enhance mobility, increase reproducibility, and realize a thin gate insulating film, by enabling flattening the surface form of a gate electrode independently of a substratum.

CONSTITUTION: After an amorphous silicon film 2 is formed on an insulating film 1, P-type impurities are introduced in the silicon film 2. After an island type gate electrode 3 is formed by patterning the amorphous silicon layer 2, a gate insulating film 4 is formed on the whole surface containing the gate electrode 3, and the impurities introduced in the gate electrode 3 are diffused by heat treatment at 800°C for 30 minutes. After a polycrystalline silicon layer 5 is formed on the whole surface, an island type active layer 5 is formed by patterning the polycrystalline silicon layer 5.