Fabrication methods of a high frequency (sub-micron gate length) operation of AlInGaN/InGaN/GaN MOS-DHFET, and the HFET device resulting from the fabrication methods, are generally disclosed. The method of forming the HFET device generally includes a novel double-recess etching and a pulsed deposition of an ultra-thin, high-quality silicon dioxide layer as the active gate-insulator. The methods of the present invention can be utilized to form any suitable field effect transistor (FET), and are particular suited for forming high electron mobility transistors (HEMT).