Channel doping is an effective method for controlling V.sub.th, but if V.sub.th shifts to the order of -4 to -3 V when forming circuits such as a CMOS circuit formed from both an n-channel TFT and a P-channel TFT on the same substrate, then it is difficult to control the V.sub.th of both TFTs with one channel dope. In order to solve the above problem, the present invention forms a blocking layer on the back channel side, which is a laminate of a silicon oxynitride film (A) manufactured from SiH.sub.4, NH.sub.3, and N.sub.2O, and a silicon oxynitride film (B) manufactured from SiH.sub.4 and N.sub.2O. By making this silicon oxynitride film laminate structure, contamination by alkaline metallic elements from the substrate can be prevented, and influence by stresses, caused by internal stress, imparted to the TFT can be relieved.