Aiming at providing a semiconductor device capable of reducing the ON-resistance when voltage smaller than a predetermined value is applied to the base region and the drift region, and capable of increasing the ON-resistance so as to prevent thermal fracture when the voltage is not smaller than the predetermined value, and at providing a method of fabricating such semiconductor device, a P-type diffusion layer 7 is formed in an N-type drift region 2 of a semiconductor device 100, as being apart from a base region 5, wherein the diffusion layer 7 is formed in a region partitioned by lines L each extending from each of the intersections of the boundary B, between the drift region 2 and a base area 5A of the base region 5, and the side faces of a trench 15 surrounding the base area 5A of the base region 5, towards the bottom plane of the drift region 2 right under the base area 5A, while keeping an angle θ2 of 50° between the lines L and the boundary B.