In a 3-terminal package sealed semiconductor device such as a transistor or a thyristor, an insulator formed between the respective lead conductors is made of alumina ceramic. An electrode conductor base (one of the lead conductors) for mounting a semiconductor device thereon is made of a copper-tungsten sintered alloy containing 2% by volume to 48% by volume of copper. The thermal expansion coefficient of the insulator becomes substantially the same as that of the electrode conductor base, thereby elmininating a conventionally used intermediate damping member and hence providing a compact semiconductor apparatus which has a high packaging density and which can switch a large current at high speed. The respective lead conductors have predetermined sizes to achieve high-speed large-current switching.