Patent ID: 7842973

Claim:
A semiconductor device comprising: a first conduction type first semiconductor layer buried in a semiconductor substrate; a first conduction type second semiconductor layer disposed above the first semiconductor layer; a first conduction type third semiconductor layer disposed above the second semiconductor layer and containing, at a portion thereof, a material having a smaller forbidden band width than that of the semiconductor substrate; and a second conduction type fourth semiconductor layer disposed above the third semiconductor layer and having a smaller forbidden band width than that of the semiconductor substrate, and a first conduction type fifth semiconductor layer disposed above the fourth semiconductor layer, wherein the fourth semiconductor layer has a region where: the forbidden band width increases stepwise or continuously toward the third semiconductor layer; the forbidden band width at the interface between the third semiconductor layer and the fourth semiconductor layer is larger than the smallest forbidden band width in the inside of the fourth semiconductor layer; and the forbidden band width is constant near the region where the forbidden band width is smallest in the inside of the fourth semiconductor layer.