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.