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Electrical properties of Ga2O3/GaAs interfaces with GdGaO cap dielectrics used in recent enhancement-mode GaAs-based NMOSFETs which perform in line with theoretical model predictions are presented. Capacitors with GdGaO thickness ranging from 3.0 to 18 nm (0.9 les EOT les 3.9 nm) have been characterized by capacitance-voltage measurements. Midgap interface state density Dit, effective workfunction phim, fixed charge Qf, dielectric constant kappa, and low field leakage current density are 2 times1011 cm-2 middoteV-1, 4.93 eV, -8.9 times1011 cm-2, 19.5, and 10-9- 10-8 A/cm2, respectively. The presence of interfacial Gd was confirmed to dramatically degrade electrical interface properties. The data illuminate the intimate interplay between heterostructure and interface engineering to achieve optimum MOSFET operation.