Patent ID: 8049251

Claim:
A semiconductor device comprising: a semiconductor substrate; a semiconductor film formed on the semiconductor substrate; a gate formed on an insulator film on the semiconductor film; a pair of first impurity diffusion regions formed by doping the semiconductor film on both side of the gate with a first impurity; and a pair of second impurity diffusion regions which is formed apart from the pair of first impurity diffusion regions and formed by doping both side of the gate with a second impurity which is of the conductivity type opposite to that of the first impurity, wherein the semiconductor film has a heterojunction structure formed by two layers of semiconductors of different types in which a third impurity having the same conductivity type as that of the second impurity exhibits different diffusion coefficients, and the third impurity is distributed with a higher concentration in one of the semiconductor layers than in the other of the semiconductor layers, wherein an impurity surface layer region is formed by doping a surface layer of the semiconductor substrate with the second impurity, and the second impurity diffusion region is formed in the impurity surface layer region, the impurity surface layer region is formed below the semiconductor film, and an upper surface of the impurity surface layer region is in contact with a lower surface of the semiconductor film, and in the pair of the second impurity diffusion regions, a first side and a second side of the second impurity diffusion regions are formed of a same impurity, the first side of the second impurity diffusion region is deeper than the second side of the second impurity diffusion region.