It is well known that the native oxide on GaAs is responsible for Fermi level pinning and has hindered the development of GaAs MOSFETs. We report significant improvements in the electrical characteristics of Au/SixNy/n-GaAs structures with NH3 plasma treatment of GaAs prior to plasma enhanced chemical vapor deposition (PECVD) of a SixNy dielectric film followed by annealing at 450 degreesC. The variation of electrical properties was studied with NH3/SiH4 ratios of 1.3, 4, 12.67, 25 and 40 for nitride deposition. It was observed that N-rich dielectric films gave the lowest interface state density. Au/Si(x)Ny/n-GaAs NUS structures were thus fabricated with interface state density of 1.1 x 10(11) eV(-1) cm(-2) as determined from G-V measurements. Post-deposition annealing showed marked improvement in device characteristics with decrease in frequency dispersion, conductance and hence interface state density as revealed from C-V to G-V measurements. The C-t studies were also carried out to determine bulk minority carrier lifetime, which was found to remain constant at I ns before and after annealing.