SEMICONDUCTOR DEVICE AND METHOD FOR MANUFACTURING THE SEMICONDUCTOR DEVICE

A semiconductor device which can make the generation of gate parasitic oscillations more difficult than a semiconductor device of the related art is provided. The semiconductor device includes: a drift layer which is constituted of a reference concentration layer and a low concentration layer; a gate electrode structure; a pair of source regions, a pair of base regions, and depletion-layer extension regions which are formed in the reference concentration layer below the base regions, wherein the depletion-layer extension regions are formed such that a lower surface of the depletion-layer extension region is deeper than a boundary between the low concentration layer and the reference concentration layer and projects into the low concentration layers, and a dVDS/dt-decreasing diffusion layer which contains an n-type impurity at a concentration higher than the concentration of the impurity which the reference concentration layer contains and decreases dVDS/dt when the semiconductor device is turned off is formed on a surface of the reference concentration layer.