To improve field effect mobility of an inverted-staggered TFT using amorphous silicon. In an inverted-staggered TFT, a thin amorphous semiconductor layer which is made to have n-type conductivity is formed between a gate insulating film and an amorphous semiconductor layer. By depositing an amorphous semiconductor layer after a substrate over which up to a gate insulating film is formed is exposed to an atmosphere which contains a phosphine gas in a small amount, an amorphous semiconductor layer which contains phosphorus is formed during the early stage of deposition of the amorphous semiconductor layer. The thus obtained amorphous semiconductor layer has the concentration peak of phosphorus around the surface of the gate insulating film.