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In this brief, AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) using an electron-beam-evaporated praseodymium oxide layer (Pr2O3) in a high-oxygen-flow environment during the gate-dielectric-layer formation was studied. By adjusting the oxygen flow rate in an electron-beam evaporator chamber, the highest Pr content in Pr2O3 occurred at 15 sccm. Moreover, the Pr2O3 thin film also achieved a good thermal stability after 400-degC, 600-degC, and 800-degC postdeposition annealing due to its high-binding-energy (933.2 eV) characteristics. The gate leakage current can be improved significantly by inserting this high- k dielectric layer, and meanwhile, the power-added efficiency can be enhanced up to 5%. Experimental results have also shown that Pr2O3 MOS-HEMTs outperformed the standard GaN HEMTs in output power density and in pulsed-mode operation. These high-performance electron-beam-evaporated Pr2O3 high-k AlGaN/GaN MOS-HEMTs are suitable for high-volume production due to its in situ insulator and metal-gate deposition in the same chamber.