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In this paper, we have demonstrated the optimized device performance in the Γ-shaped gate AlGaN/AlN/GaN metal oxide semiconductor high electron mobility transistor (MOS-HEMT) by incorporating aluminum into atomic layer deposited (ALD) HfO₂ and comparing it with the commonly used HfO₂ gate dielectric with the N₂ surface plasma treatment. The inclusion of Al in the HfO₂ increased the crystalline temperature (~1000 °C) of hafnium aluminate (HfAlO_X) and kept the material in the amorphous stage even at very high annealing temperature (>800 °C), which subsequently improved the device performance. The gate leakage current (I_G) was significantly reduced with the increasing post deposition annealing (PDA) temperature from 300 to 600 °C in HfAlO_X-based MOS-HEMT, compared to the HfO₂-based device. In comparison with HfO₂ gate dielectric, the interface state density (D_it) can be reduced significantly using HfAlO_X due to the effective passivation of the dangling bond. The greater band offset of the HfAlO_X than HfO₂ reduces the tunneling current through the gate dielectric at room temperature (RT), which resulted in the lower I_G in Γ-gate HfAlO_X MOS-HEMT. Moreover, I_G was reduced more than one order of magnitude in HfAlO_X MOS-HEMT by the N₂ surface plasma treatment, due to reduction of N₂ vacancies which were created by ICP dry etching. The N₂ plasma treated Γ-shaped gate HfAlO_X-based MOS-HEMT exhibited a decent performance with I_DMAX of 870 mA/mm, G_MMAX of 118 mS/mm, threshold voltage (V_TH) of −3.55 V, higher I_ON/I_OFF ratio of approximately 1.8 × 10⁹, subthreshold slope (SS) of 90 mV/dec, and a high V_BR of 195 V with reduced gate leakage current of 1.3 × 10⁻¹⁰ A/mm.