Effect of Sputtered-Al2O3 Layer Thickness on the Threshold Voltage of III-Nitride MIS-HEMTs

Dependence of threshold voltage (V_Th) on oxide thickness (t_ox) for GaN-based metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) using sputter deposited Al₂O₃ as gate dielectric is studied in detail. Different III-nitride (III-N) heterostructures (AlGaN/GaN and AlInN/GaN) with/without GaN cap layer were used for fabricating these MIS devices. Interestingly, for all the sets of devices, a positive shift in V_Th was observed initially with a increase in t_ox, followed by a negative shift of the same. A comprehensive analytical model has been proposed to explain the variation of V_Th with t_ox and has been shown to match the experimental data for MIS-HEMTs fabricated on different heterostructures and with different values of t_ox. This model allows the extraction of different charge components in the oxide or at oxide/III-N interface. Normally OFF AlInN/GaN MIS-HEMTs with V_Th of +0.67 V have been demonstrated with the optimized tox of sputtered Al₂O₃.