FIG. 3 is the result of investigating the thickness of the silicon oxynitride film (A) and the silicon oxynitride film (B), which compose the blocking layer 102, and the Vshift of the n-channel TFT. The data plotted in FIG. 3 are all for test pieces which were not channel doped, and the fact that there is a combination of an ideal film thickness in order to have the Vshift value within a fixed ran