Influence of process technology on DC-performance of GaN-based HFETs

This work reports on the influence of the surface and the gate length on the performance of AlGaN/GaN based Hetero Field Effect Transistors (HFETs). Differently NH4Sx treated surfaces result in variation of the drain current IDmax of more then 100%. Gate recessing by photoelectrochemical treatment changes the threshold voltage Vth but affects the drain current little. Next, the reduction of the gate length increases the IDmax further by more than 60%. The IDmax values for the transistors are 350 mA mm-1 for the HN4Sx-treated, 850 mA for the untreated, and 1.43 A mm-1 for the one with a 0.2 μm gate length. The corresponding transconductances gm are 66, 150, and 280 mS mm-1, respectively. Surface analysis with Auger Electron Spectroscopy (AES) and contact characterization (TLM) reveals, that the NH4Sx treatment removes the native oxide and increases the contact resistance as well. Therefore we attribute the increase of IDmax and gm mainly to a beneficial behavior of gallium-oxide at the surface on the sheet carrier density nS of the 2DEG at the heterointerface.