A semiconductor substrate includes a high-resistivity silicon carbide layer and a gallium nitride epitaxial layer. The gallium nitride epitaxial layer is formed on a surface, a thickness of the gallium nitride epitaxial layer is less than 2 μm, and a full width at half maximum (FWHM) of an X-ray diffraction analysis (002) plane is less than 100 arcsec. The thickness of the high-resistivity silicon carbide layer ranges from 20 μm to 50 μm. The surface of the high-resistivity silicon carbide layer has an angle ranging from 0° to +/−8° with respect to a (0001) plane. The micropipe density (MPD) of the high-resistivity silicon carbide layer is less than 0.5 ea/cm2, the basal plane dislocation (BPD) of the high-resistivity silicon carbide layer is less than 10 ea/cm2, and the threading screw dislocation (TSD) of the high-resistivity silicon carbide layer is less than 500 ea/cm2.