A first aspect of the invention is to realize a power
amplifier having high power adding efficiency and high
power gain at low cost. For that purpose, in a
semiconductor device using an emitter top
heterojunction bipolar
transistor formed above a
semiconductor substrate and having a planar shape in a ring-like shape, a structure is provided in which a base
electrode is present only on an inner side of a ring-like emitter-base junction region. In this way, as a result of enabling to reduce base / collector junction
capacitance per unit emitter area without using a collector top structure having complicated fabricating steps, a
semiconductor device having high power adding efficiency and high-
power gain and suitable for a power
amplifier can be realized. A second aspect of the application is to provide a power
amplifier enabling to reduce temperature dependency of
power gain. For that purpose, in a multistage power amplifier including a first amplifier circuit 2 having one or more of bipolar transistors connected in parallel and arranged above a first semiconductor substrate and a second amplifier circuit 3 having one or more of bipolar transistors connected in parallel and arranged above a second semiconductor substrate, the bipolar
transistor used in the first amplifier circuit 2 is provided with an emitter shape having a planar shape in a rectangular shape and the bipolar
transistor used in the second amplifier circuit 3 is provided with an emitter shape in, for example, a ring-like shape and a base
electrode thereof is present only on the inner side of the ring-like emitter.