A semiconductor device raises the maximum oscillation frequency fmax
of the bipolar transistor. The stopper dielectric layer is formed on the substrate
to cover the transistor section and the isolation dielectric. The interlayer dielectric
layer is formed on the stopper dielectric layer. The base contact plug, which is
formed in the interlayer dielectric layer, is located over the isolation dielectric
in such a way as to contact the graft base region near its bottom end corner. Therefore,
the base contact needs not to entirely overlap with the graft base region, which
means that the graft base region can be narrowed without increasing the base resistance
Rb and that the collector-base capacitance Ccb is reduced.
Also, electrical short circuit between the graft base region and the collector
region can be effectively suppressed by the stopper dielectric layer.
