| 摘要 |
An epitaxial wafer including nitride-based semiconductor layers usable for a hetero-junction field effect type transistor, includes a first buffer layer of AlN or AlON, a second buffer layer of AlxGa1-xN having its Al composition ratios decreased in a stepwise fashion, a third buffer layer including a multilayer of repeatedly stacked AlaGa1-aN layers/AlbGa1-bN layers disposed on the second buffer layer, a GaN channel layer, and an electron supply layer in this order on a Si substrate, wherein the Al composition ratio x in the uppermost part of the second buffer layer is in a range of 0≦̸x≦̸0.3.
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