Patent ID: 8643095

Claim:
A semiconductor transistor device comprising: a drift layer of a first conductivity type; a base layer of a second conductivity type provided on the drift layer, the base layer containing an impurity of the first conductivity type and an impurity of the second conductivity type, an impurity concentration of the second conductivity type in the base layer being higher than an impurity concentration of the first conductivity type in the base layer; a source layer of a first conductivity type selectively provided on a surface of the base layer; a gate electrode and a field plate electrode under the gate electrode surrounded by an insulating film, a first end of the gate electrode being located at a level of the source layer, and a second end of the gate electrode being located at a level of the drift layer; a drain electrode electrically connected to the drift layer; and a source electrode electrically connected to the source layer, wherein: the field plate electrode is electrically connected to the source electrode; the impurity concentration of the first conductivity type contained in the base layer is lower than an impurity concentration of the first conductivity type contained in the drift layer; and the impurity concentration of the first conductivity type contained in the drift layer is not less than 1×10 16 (atoms/cm 3 ).