a substrate;

a nucleation layer comprising a composition that includes a first element and disposed on and forming an interface with the substrate;

a buffer layer comprising a III-V compound which includes the first element, the buffer layer disposed on and forming an interface with the nucleation layer, wherein 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, wherein a first oscillation rate between a first reference point and a second reference point within the buffer layer is greater than a second oscillation rate between the second reference point and a third reference point within the buffer layer;

a first nitride-based semiconductor layer disposed on and forming an interface with the buffer layer, wherein the concentration of the first element within the buffer layer first oscillates with respect to the first reference point so that peaks and troughs of the concentration converge toward a positive steady value, and then the concentration drops to 0 at the interface between the buffer layer and the first nitride-based semiconductor layer;

a second nitride-based semiconductor layer disposed on the first nitride-based semiconductor layer and having a bandgap greater than a bandgap of the first nitride-based semiconductor layer, so as to form a heterojunction therebetween with a two-dimensional electron gas (2DEG) region; and

two or more source/drain (S/D) electrodes and a gate electrode disposed over the second nitride-based semiconductor layer, wherein the gate electrode is between the S/D electrodes.