Typically GaN metal-oxide-semiconductor heterojunction-field-effect transistors (MOS-HFETs) have used two separate dielectrics for the gate and access regions. However, as this article shows, with proper gate-stack engineering, a unified dielectric solution can be achieved for the transistor. HfO₂ dielectrics were deposited by atomic layer deposition (ALD). Two types of oxidants were investigated, namely, water (H₂O) and ozone (O₃). It was found that MOS-HFETs with O₃ oxidant yielded lower threshold voltage (<inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula>) shifts, higher maximum drain current (<inline-formula> <tex-math notation="LaTeX">${I}_{\text {DS,max}}$ </tex-math></inline-formula>) of 340 mA/mm, 20% lower ON-resistance (<inline-formula> <tex-math notation="LaTeX">${R}_{ {\mathrm {\scriptscriptstyle {ON}}}}$ </tex-math></inline-formula>), higher peak transconductance at 112.66 mS/mm, lower hysteresis, and lower gate leakage (<inline-formula> <tex-math notation="LaTeX">${5.4} \times {10}^{-{6}}$ </tex-math></inline-formula> A/cm²) compared to water oxidant based MOS-HFETs with <inline-formula> <tex-math notation="LaTeX">${I}_{\text {DS},\text {max}}$ </tex-math></inline-formula> of 240 mA/mm, 81.38 mS/mm peak transconductance, and <inline-formula> <tex-math notation="LaTeX">${1.7} \times {10}^{-{4}}$ </tex-math></inline-formula> A/cm² gate leakage. DC/RF dispersion tests showed MOS-HFETs with O₃ oxidant had ~200<inline-formula> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> better current collapse recovery. Temperature characterization and reliability test results, such as high-temperature reverse bias (HTRB), are published for the first time on ALD-HfO₂/AlGaN/GaN MOS-HFETs using tetrakis(dimethylamino)hafnium (TDMAH) and O₃ precursor. Using an ozone oxidant provided more stability (i.e., less variability in <inline-formula> <tex-math notation="LaTeX">${R}_{ {\mathrm {\scriptscriptstyle {ON}}}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula>) as a function of temperature. Finally, when devices were electrically stressed in the OFF-state, the HTRB test showed minimal <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> drift (<0.5 V) in the case of O₃ oxidant versus much larger <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> drift (2.5 V) in the case of H₂O oxidant.