A semiconductor device comprising a plurality of heterojunction bipolar transistors with their base layer made of GaAsSb or InGaAs, a GaAs substrate, and a buffer layer placed between the base layer and the substrate is fabricated. The substrate and the buffer layer that lie directly under the intrinsic regions of a part or all of the plurality of heterojunction bipolar transistors are removed. Thereby, a semiconductor device using HBTs that can operate with a power supply voltage of 2 V or below can be provided at reduced cost as a well-reliable product, and a power amplifier with high power conversion efficiency can be provided.