Metal-oxide-semiconductor (MOS) capacitors with sputtering-deposited Gd-doped HfO2(HGO) high k gate dielectric thin films and ALD-derived Al2O3 interfacial passivation layer were fabricated on GaAs substrates. The effects of the passivation layer and the forming gas annealing (FGA) temperature were explored by studying the interfacial chemical bonding states and electrical properties of HGO/GaAs and HGO/Al2O3/GaAs gate stacks via x-ray photoelectron spectroscopy (XPS), capacitance-voltage (C-V), and leakage current density-voltage (J-V) measurements. Results indicated that the MOS capacitors performances were enhanced by performing FGA. The electrical analysis revealed that the 300 degrees C-annealed Al/HGO/GaAs/Al MOS capacitor with 20 cycles Al2O3 passivation layer experienced improved electrical properties, with a dielectric constant of 44, a flat band voltage of 0.64 V, a hysteresis of 0.02 V corresponding to the oxide charge density of -6.2 x 10(12) cm(2), border trapped oxide charge density of -3.02 x 10(11) cm(2), a leakage current density 5.87 x 10(-6) A/cm(2) at a bias voltage of 2 V. The low temperature (77-300 K) dependent detailed current conduction mechanisms (CCMs) of the 300 degrees C-annealed MOS capacitor at low temperatures were also systematically investigated. The optimized interface chemistry and the excellent electrical properties suggested that HGO/Al2O3/GaAs potential gate stacks could be applied in future III-V-based MOSFET devices.