A semiconductor device includes a circuit board formed of an insulator substrate and having conductor patterns on both surfaces thereof, a semiconductor chip bonded to the circuit board with one of the conductor patterns therebetween, and a radiator base bonded to the circuit board with a solder layer through the other of the conductor patterns therebetween for conducting heat generated in the semiconductor chip to an outside device. The radiator base is formed of a material having anisotropic thermal conductivity so that the radiator base has thermal conductivity in a direction perpendicular to a bonding plane between the radiator base and the circuit board higher than that along the bonding plane. The radiator base has thermal expansion coefficient along a bonding plane with the circuit board different from that along the bonding plate of the insulator substrate by a predetermined value.