Thermal stability of HfO$_2$|AlGaN|GaN normally-Off transistors with Ni|Au and Pt gate metals

A hybrid gate approach, including a two-step gate recess and a sputtered HfO$_2$ layer, is employed to fabricate HfO$_2$|AlGaN|GaN normally-Off high electron mobility transistors (HEMTs). Ni|Au and Pt are used as gate metals in the studied metal–oxide–semiconductor (MOS)-type devices. The two-step gate recess approach can effectively deplete the two-dimensional electron gas density in the channel. The studied MOS-type structure demonstrates a lower gate leakage and significantly positive gate threshold voltage. Attributed to the high work function of Pt metal, the device with Pt gate metal exhibits excellent thermal stability, including gate leakage, high saturation current, transconductance, On/Off current ratio, and threshold voltage, measured from 300 to 500 K, as compared with the device with Ni|Au gate metal. Furthermore, the studied devices show superior behaviors as compared with other devices.