Abstract: A semiconductor device includes a substrate of a first conductivity type. A first layer of a second conductivity type is formed on one surface of the substrate of the first conductivity type. A second layer of the first conductivity type is formed on the first layer of the second conductivity type. A third layer of the first conductivity type is selectively formed on the other surface of the substrate of the first conductivity type. A fourth layer of the second conductivity type is formed on the other surface of the substrate of the first conductivity type. The vertical dimension of the fourth layer falls within a range of 5 to 20 .mu.m and is smaller than that of the first layer.