A semiconductor device has an external wiring for GND formed over an underside surface of a wiring substrate. A plurality of via holes connecting to the external wiring for GND are formed to penetrate the wiring substrate. A first semiconductor chip of high power consumption, including HBTs, is mounted over a principal surface of the wiring substrate. The emitter bump electrode of the first semiconductor chip is connected in common with emitter electrodes of a plurality of HBTs formed in the first semiconductor chip. The emitter bump electrode is extended in a direction in which the HBTs line up. The first semiconductor chip is mounted over the wiring substrate so that a plurality of the via holes are connected with the emitter bump electrode. A second semiconductor chip lower in heat dissipation value than the first semiconductor chip is mounted over the first semiconductor chip.