An IGBT is disclosed which has a set of inside trenches and an outside trench formed in its semiconductor substrate. The substrate has emitter regions adjacent the trenches, a p-type base region adjacent the emitter regions and trenches, and an n-type base region comprising a first and a second subregion contiguous to each other. The first subregion of the n-type base region is contiguous to the inside trenches whereas the second subregion, less in impurity concentration than the first, is disposed adjacent the outside trench. Breakdown is easier to occur than heretofore adjacent the inside trenches, saving the device from destruction through mitigation of a concentrated current flow adjacent the outside trench.