We report fabrication of self-aligned enhancement-mode GaAs MOSFETs employing a simple ex situ chemical clean. The role of precursor chemistry in removing GaAs native oxides as well as the impact of different ex situ chemical cleans on the properties of the interface between various atomic-layer-deposited high-k gate dielectrics and GaAs substrates were examined using x-ray photoelectron spectroscopy and Raman spectroscopy. The material characterization results indicate the effectiveness of ex situ sulfur passivation of GaAs surface prior to high-k deposition using ammonium sulfide solutions in improving the interface properties between high-k layers and GaAs substrates. Moreover, an appropriate choice of precursor chemistry for reduction of GaAs native oxides appears to play a crucial role in mitigating the Fermi level pinning at the interface. A maximum drive current of 4.5 μA/μm at a gate overdrive of 2.5V was obtained for a MOSFET with a gate length of 20μtn.