A semiconductor substrate and a method of its manufacture has a semiconductor substrate having a carbon concentration in a range of 6.0×1015 to 2.0×1017 atoms/cm3, both inclusively. One principal surface of the substrate is irradiated with protons and then heat-treated to thereby form a broad buffer structure, namely a region in a first semiconductor layer where a net impurity doping concentration is locally maximized. Due to the broad buffer structure, lifetime values are substantially equalized in a region extending from an interface between the first semiconductor layer and a second semiconductor layer formed on the first semiconductor layer to the region where the net impurity doping concentration is locally maximized. In addition, the local minimum of lifetime values of the first semiconductor layer becomes high. It is thus possible to provide a semiconductor device having soft recovery characteristics, in addition to high-speed and low-loss characteristics, while suppressing a kinked leakage current waveform.