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This work investigates, for the first time, wide-gap Al0.21Ga0.79N channel metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with In0.12Al0.76Ga0.12N barrier/buffer and drain field-plate (DFP) designs. High-k and wide-gap Al2O3 was grown as the gate oxide and surface passivation by using non-vacuum ultrasonic spray pyrolysis deposition (USPD) technique. A control device having the same epitaxial layers, except with In0.12Al0.88N barrier/buffer was studied in comparison. Enhanced spontaneous polarization effect, improved interfacial quality, and enhanced carrier confinement have been achieved by using the In0.12Al0.76Ga0.12N barrier/buffer design, which has successfully resulted in improved carrier transport, increased electron concentration, and high current densities. The present In0.12Al0.76Ga0.12N/AlN/Al0.21Ga0.79N MOS-HFET design with (without) DFP design has demonstrated superior maximum drain-source current density <inline-formula> <tex-math notation="LaTeX">$(I_{DS, max})$ </tex-math></inline-formula> of &#x003E;1 (&#x003E;1) A/mm at <inline-formula> <tex-math notation="LaTeX">$V_{DS}\,\,=$ </tex-math></inline-formula> 20 V, high saturated drain-source current density at <inline-formula> <tex-math notation="LaTeX">$V_{GS}\,\,=$ </tex-math></inline-formula> 0 V <inline-formula> <tex-math notation="LaTeX">$(I_{DSS0})$ </tex-math></inline-formula> of 791.1 (755) mA/mm, and low specific on-resistance <inline-formula> <tex-math notation="LaTeX">$(R_{on, sp})$ </tex-math></inline-formula> of 2.83 (2.81) <inline-formula> <tex-math notation="LaTeX">$\text{m}\Omega $ </tex-math></inline-formula>/cm 2. High device figure-of-merit (FOM) on <inline-formula> <tex-math notation="LaTeX">$BV_{DS}^{2}/R_{on, sp}$ </tex-math></inline-formula> of 93.7 (75.4) MW/cm 2 was also obtained with the three-terminal on-state drain-source breakdown voltage <inline-formula> <tex-math notation="LaTeX">$(BV_{DS})$ </tex-math></inline-formula> of 515 (460) V. The present design is promisingly advantageous to high-current and high-voltage power-switching circuit applications.