To improve mobility as well as obtain a desired threshold voltage, by forming films on a plurality of semiconductor layers which are formed in different film formation conditions, and allowing the localized level density of electrons of the semiconductor layer in contact with a gate insulation layer to be different from that of a semiconductor layer adjacent to back channel side.

A film is formed on a gate insulation film 2 and an intrinsic amorphous silicon layer as a channel layer sequentially and continuously by plasma CVD method. When the intrinsic amorphous silicon layer is subject to film formation through the plasma CVD method, the film formation condition for a semiconductor layer in contact with the gate insulation film 2 is made to be different from that of a semiconductor layer on the side of back channel, and a semiconductor layer 3a with a low localized level density and a semiconductor layer 3b with a high localized level density are formed. Here, the back channel is a part which is not in contact with the gate insulation film 2 and is not in contact with a source electrode and a drain electrode as well, among a channel layer.