An element belonging to Group 15 of the periodic table, typically, phosphorus or arsenic is added in the case of forming an n-channel TFT. An element belonging to Group 13, typically, boron is added in the case of forming a p-channel TFT. The second semiconductor film is formed by a plasma CVD method using a silicide gas mixed with a gas including an element belonging to Group 13 or 15 such as bor