A semiconductor device includes a nucleation layer, a buffer layer, a first nitride-based semiconductor layer, a second nitride-based semiconductor layer, S/D electrodes, and a gate electrode. The nucleation layer includes a composition that includes a first element. The buffer layer includes a III-V compound which includes the first element. The buffer layer has a concentration of the first element oscillating within the buffer layer, such that the concentration of the first element varies as an oscillating function of a distance within a thickness of the buffer layer. A first oscillation rate between a first reference point and a second reference point within the buffer layer is less than a second oscillation rate between the second reference point and a third reference point within the buffer layer. The first and second nitride-based semiconductor layer, S/D electrodes, and a gate electrode are disposed on the buffer layer.