Devices and methods of a field effect transistor device that include a source, a gate and a drain. The transistor includes a semiconductor region position is under the source, the gate and the drain. Such that the semiconductor region can include a gallium nitride (GaN) layer and an III Nitride (III-N) layer. Wherein the GaN layer includes a band gap, and the III-N layer includes a band gap. Such that the III-N layer band gap is higher than the GaN layer band gap. A sub-region of the semiconductor region is located underneath the gate and is doped with Mg ions at selective locations in the sub-region.