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A novel surface passivation technology employing silane (SiH4) and ammonia (NH3) was demonstrated to realize high-quality metal-gate/high-k dielectric stack on GaAs. In addition to ex situ cleaning/passivation and in situ vacuum anneal to remove the native oxide on GaAs, the key improvements reported in this letter include the introduction of NH3 in a SiH4 passivation to form a SiN passivation layer that protects the GaAs surface from exposure to the oxidizing ambient during high- k dielectric deposition. Negligible As-O and Ga-O bonds were found. This passivation technology was integrated in a metal-organic chemical-vapor deposition tool. Inversion-type GaAs n-MOSFETs were fabricated with the SiH4 and NH3 passivation technology, showing good electrical characteristics with a peak effective mobility of 1920 cm2/V middots, an I on/I off ratio of ~ 105, and a subthreshold swing of ~ 98 mV/dec, in surface-channel GaAs MOSFETs with a gate length of 2 mum.