formed in a part of the base region is an impurity diffusion region extending in a vertical direction and having an impurity concentration lower than that in the other portion of the base region . by the formation of the impurity diffusion region , the depletion layer is extended toward the base region so as to improve the breakdown voltage . the impurity diffusion region is formed by forming a trench in a part of the base region , a conductive film being buried in the trench , followed by introducing by ion implantation an impurity of the conductivity type equal to that in the base region into the side wall and the bottom of the trench in a concentration lower than that in the base region and subsequently diffusing the implanted impurity ions . the impurity diffusion region thus formed permits relaxing the electric field concentration on the corner portion of the gate trench and on the extended portion of the base region so as to improve the breakdown voltage .