Investigation of threshold voltage shift and gate leakage mechanisms in normally off AlN/Al0.05Ga0.95N HEMTs on Si substrate

In this study, normally off AlN/Al0.05Ga0.95N high-electron-mobility transistors (HEMTs) on a Si substrate were fabricated by adjusting the surface states of the heterostructure. The device exhibited an extremely low reverse gate leakage current of 10−7 mA/mm due to the high Schottky-barrier height for Ni/Au on AlN/Al0.05Ga0.95N. A high ION/IOFF of 108 and a low subthreshold slope of 63 mV/decade were attained for this device. Moreover, breakdown voltages of 665 V and 1000 V were reached in these devices, with a gate-to-drain distance of 26 µm, for a grounded substrate and a floating substrate, respectively. In order to evaluate the reliability of the device, bias-induced and temperature-induced threshold-voltage-instabilities were investigated. The threshold voltage of the device shifted with gate bias stress due to electrons in the channel trapped by bulk traps. Thermally activated electrons releasing from traps decreased the threshold voltage with increasing measurement temperature. This indicates that the reliability in the threshold-voltage stability for the normally off device is dominated by the deep traps in epitaxially grown AlN/Al0.05Ga0.95N. Finally, the gate leakage mechanisms in AlN/Al0.05Ga0.95N HEMTs were investigated. The reverse and forward gate leakage was dominated by Poole–Frenkel tunneling and thermionic emission, respectively.