A semiconductor device includes a low resistance semiconductor substrate, a high
resistance semiconductor layer formed on the substrate, an insulation layer formed
on the semiconductor layer, and a transistor element composed of a collector region,
abase region, and an emitter region formed in the semiconductor layer. The device
further includes an emitter electrode formed in the insulation layer to be connected
to the emitter region, a sub-emitter electrode formed in the insulation layer connected
to the emitter electrode, a low resistance impurity-diffusion region formed in
the semiconductor layer such that the sub-emitter electrode is connected to the
substrate through the impurity-diffusion region, a base electrode formed in the
insulation layer to be connected to the base region, and a base-bonding pad formed
on the insulation layer to be connected to the base electrode. The base-bonding
pad is placed on the insulation layer above the impurity-diffusion region to be
at least partially encompassed with the impurity-diffusion region.
