A normally-off, heterojunction field effect transistor includes an intrinsic cubic-phase gallium nitride (c-GaN) substrate and an aluminum gallium nitride (AlGaN) capping layer disposed on the intrinsic c-GaN substrate. The AlGaN capping layer includes a first sublayer of intrinsic c-phase AlxGa1-xN disposed on the c-GaN substrate, wherein the first sublayer is of a first thickness; a second sublayer of doped c-phase AlxGa1-xN disposed on the first sublayer, and wherein the second sublayer is of a second thickness and is doped with a dopant. An insulating layer is disposed on the AlGaN capping layer, wherein the insulating layer is of a fourth thickness. A source electrode, a drain electrode, and a gate electrode are positioned adjacent to and on top of the insulating layer, respectively.