In conventional mesa-type npn bipolar transistors, the improvement of a current
gain and the
miniaturization of the
transistor have been unachievable simultaneously as a result of a trade-off being present between
lateral diffusion and recombination of the electrons which have been injected from an emitter layer into a base layer, and a high-density base
contact region—emitter mesa distance. In contrast to the above, the present invention is provided as follows:The gradient of
acceptor density in the
depth direction of a base layer is greater at the edge of an emitter layer than at the edge of a collector layer. Also, the distance between a first mesa structure including the emitter layer and the base layer, and a second mesa structure including the base layer and the collector layer, is controlled to range from 3 μm to 9 μm. In addition, in order for the above to be implemented with high
controllability, the base layer is formed of a first p-type base layer having an
acceptor of uniform density, and a second p-type base layer whose density is greater than the uniform
acceptor density of the first base layer while having a gradient in the
depth direction of the second base layer. These features produce the advantageous effect that it is possible to provide a high-temperature adaptable, power-switching bipolar
transistor that ensures a current
gain high enough for practical use and is suitable for
miniaturization.