The high-dielectric-constant (high-k) gadolinium oxide layer (Gd2O3) and praseodymium oxide layer (Pr2O3) are demonstrated as gate dielectric insulator materials in GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) using in situ high oxygen flow rate electron-beam deposition technology. The dielectric constants of the Gd2O3 and Pr2O3 layers developed in this study were 9.2 and 9.8, respectively. The Schottky gate turn-on voltages of GaAs MOSFETs with Gd2O3 and Pr2O3 insulators were 2.23 and 2.25 V, respectively, representing an improvement on the conventional p-type high electron mobility transistors (0.85 V). Moreover, the Gd2O3 MOSFETs had a higher thermal stability and thermal linearity than the Pr2O3 MOSFET (temperature range 100-400 K) due to its high binding energy, as revealed by X-ray photoelectron spectroscopy. (c) 2008 The Electrochemical Society. [DOI: 10.1149/1.2988050] All rights reserved.