Semiconductor device and manufacturing method thereof

A semiconductor device comprising:

• a semiconductor substrate having an upper surface and a lower surface, wherein
• in a depth direction connecting the upper surface and the lower surface of the semiconductor substrate, a donor concentration distribution includes:
• a first donor concentration peak placed at a first depth;
• a second donor concentration peak placed at a second depth between the first donor concentration peak and the upper surface, the second donor concentration peak having a lower concentration than the first donor concentration peak;
• a flat region placed between the first donor concentration peak and the second donor concentration peak, wherein the donor concentration distribution in the flat region is substantially flat and a thickness of the flat region in the depth direction is 10% or more of a thickness of the semiconductor substrate in the depth direction;
• and
• a plurality of donor concentration peaks placed between the first donor concentration peak and the lower surface.