A nitride-based semiconductor substrate has a diameter of 25 mm or more, a thickness of 250 micrometers or more, a n-type carrier concentration of 1.2×1018 cm−3 or more and 3×1019 cm−3 or less, and a thermal conductivity of 1.2 W/cmK or more and 3.5 W/cmK or less. Alternatively, the substrate has an electron mobility μ [cm2/Vs] of more than a value represented by loge μ=17.7−0.288 loge n and less than a value represented by loge μ=18.5−0.288 loge n, where the substrate has a n-type carrier concentration n [cm−3] that is 1.2×1018 cm−3 or more and 3×1019 cm−3 or less.