A semiconductor device includes a semiconductor layer including a base region of a second conductive type formed in a first surface of the semiconductor layer, an emitter region of the first conductive type formed in the base region, a buffer layer of the first conductive type formed on a second surface of the semiconductor layer, and a collector layer of the second conductive type formed on the buffer layer. The buffer layer has a maximal concentration of the first conductive type impurity of approximately 5 ×1015 cm−3 or less, and the collector layer has a maximal concentration of the second conductive type impurity of approximately 1×1017 cm−3 or more. The ratio of the maximal concentration of the collector layer to that of the buffer layer is greater than 100. The collector layer has a thickness of approximately 1 μm or more.