Abstract: A compound semiconductor substrate which inhibits the generation of a crack or a warp and is preferable for a normally-off type high breakdown voltage device, arranged that a multilayer buffer layer 2 in which AlxGa1-xN single crystal layers (0.6?X?1.0) 21 containing carbon from 1×1018 atoms/cm3 to 1×1021 atoms/cm3 and AlyGa1-yN single crystal layers (0.1?y?0.5) 22 containing carbon from 1×1017 atoms/cm3 to 1×1021 atoms/cm3 are alternately and repeatedly stacked in order, and a nitride active layer 3 provided with an electron transport layer 31 having a carbon concentration of 5×1017 atoms/cm3 or less and an electron supply layer 32 are deposited on a Si single crystal substrate 1 in order. The carbon concentrations of the AlxGa1-xN single crystal layers 21 and that of the AlyGa1-yN single crystal layers 22 respectively decrease from the substrate 1 side towards the above-mentioned active layer 3 side. In this way, the compound semiconductor substrate is produced.

Abstract: The present invention provides a compound semiconductor substrate, including: a single-crystal silicon substrate having a crystal face with (111) orientation; a first buffer layer which is formed on the single-crystal silicon substrate and is constituted of an AlxGa1-xN single crystal (0<x?1); a second buffer layer which is formed on the first buffer layer and is composed of a plurality of first unit layers each having a thickness of from 250 nm to 350 nm and constituted of an AlyGa1-yN single crystal (0?y<0.1) and a plurality of second unit layers each having a thickness of from 5 nm to 20 nm and constituted of an AlzGa1-zN single crystal (0.9<z?1), said pluralities of first and second unit layers having been alternately superposed; and a semiconductor device formation region which is formed on the second buffer layer and includes at least one nitride-based semiconductor single-crystal layer.