An improved HEMT formed from a GaN material system is disclosed which has reduced gate leakage current and eliminates the problem of current constrictions resulting from deposition of the gate metal over the step discontinuities formed over the gate mesa. One or more GaN based materials are layered and etched to form a gate mesa with step discontinuities defining source and drain regions. In order to reduce the leakage current, the step discontinuities are back-filled with an insulating material, such as silicon nitride (SiN), forming a flat surface relative to the source and drain regions, to enable to the gate metal to lay flat. By back-filling the source and drain regions with an insulating material, leakage currents between the gate and source and the gate and drain are greatly reduced. In addition, current constrictions resulting from the deposition of the gate metal over a step discontinuity are virtually eliminated.