A semiconductor device includes a semiconductor substrate, an upper diffusion region and a lower diffusion region. The semiconductor substrate has a main surface. The upper diffusion region of a first conductivity type is disposed close to the main surface of the semiconductor device. The lower diffusion region of a second conductivity type is disposed up to a position deeper than the upper diffusion region in a depth direction of the semiconductor substrate from the main surface as a reference, and has a higher impurity concentration than the semiconductor substrate. A diode device is provided by having a PN junction surface at an interface between the upper diffusion region and the lower diffusion region, and the PN junction surface has a curved surface disposed at a portion opposite to the main surface.