A process for fabricating a semiconductor at a lower crystallization temperature and yet at a shorter period of time, which comprises forming an insulator coating on a substrate; exposing said insulator coating to a plasma; forming an amorphous silicon film on said insulator coating after its exposure to said plasma; and heat treating said silicon film in the temperature range of from 400 to 650° C. or at a temperature not higher than the glass transition temperature of the substrate. The nucleation sites are controlled by selectively exposing the amorphous silicon film to a plasma or by selectively applying a substance containing elements having a catalytic effect thereto. A process for fabricating a thin film transistor using the same is also disclosed.