A semiconductor device in which a semiconductor layer is formed on an insulating substrate with a front-end insulating layer interposed between the semiconductor layer and the insulating substrate is provided which is capable of preventing action of an impurity contained in the insulating substrate on the semiconductor layer and of improving reliability of the semiconductor device. In a TFT (Thin Film Transistor), boron is made to be contained in a region located about 100 nm or less apart from a surface of the insulating substrate so that boron concentration decreases at an average rate being about 1/1000-fold per 1 nm from the surface of the insulating substrate toward the semiconductor layer.