Patent ID: 7728409

Claim:
A semiconductor device comprising: a first semiconductor layer of a first conduction type, having a first main surface and a second main surface opposed to the first main surface, a second semiconductor layer of a second conduction type, which is high in concentration compared with the first semiconductor layer and provided in contact with the first semiconductor layer at a side of the first main surface of the first semiconductor layer, a third semiconductor layer of the first conduction type, which is high in concentration compared with the first semiconductor layer and provided at a side of the second main surface of the first semiconductor layer, a fourth semiconductor layer of the first conduction type, which is high in concentration compared with the first semiconductor layer and low in concentration compared with the third semiconductor layer, and provided in contact with both the first semiconductor layer and the third semiconductor layer between the first semiconductor layer and the third semiconductor layer, a first electrode electrically connected to the second semiconductor layer, and a second electrode electrically connected to the third semiconductor layer, wherein a thickness of the fourth semiconductor layer in a direction from the first main surface to the second main surface of the first semiconductor layer is large compared with a thickness of the third semiconductor layer in the same direction, an impurity concentration of the fourth semiconductor layer is in the range from 1×10 14 atoms/cc to 1×10 15 atoms/cc, and at a position located in the fourth semiconductor layer, an integral value of the impurity concentration is equal to n c , the integral value obtained by integrating the impurity concentration from a boundary of the first semiconductor layer with the second semiconductor layer to the second main surface, where: n c =∈ s ·E c /q, ∈ s is a dielectric constant of a semiconductor, q is an elementary charge, and E c is a dielectric breakdown field strength of the semiconductor.