Transistor, method for producing same, and substrate

The invention relates to a transistor, a method for producing same, and a substrate. Disclosed is a semiconductor device (1). The semiconductor device (1) has a semiconductor layer (21-25) and a substrate (2). The semiconductor layer (21-25) constitutes at least a part of a current path, and is made of silicon carbide. The substrate (2) has a first surface (2A) supporting the semiconductor layer (21-25), and a second surface (2B) opposite to the first surface (2A). Further, the substrate (2) is made of silicon carbide having a 4H type single-crystal structure. Further, the substrate (2) has a physical property in which a ratio of a peak strength in a wavelength of around 500 nm to a peak strength in a wavelength of around 390 nm is 0.1 or smaller in photoluminescence measurement. In this way, the semiconductor device (1) is obtained to have a low on-resistance.