A semiconductor device 10 includes a first transistor 11 placed on a substrate 16, a second transistor 12 placed on the first transistor 11 via a heat radiation layer 17, a third transistor 13 placed on the substrate 16, and a fourth transistor 14 placed on the third transistor 11 via a heat radiation layer 17. The first transistor 11 has a first region corresponding to a region where the second transistor is placed, and a second region which is formed so as to surround the first region and in which the rate of area occupied by the emitter region in the base region is higher than in the first region. Likewise the first transistor 11, the third transistor 13 has a region in which the rate of area occupied by the emitter region in the base region is varied.