The properties of different rectifying metallizations (Al, Ti/Pt, WNx) on GaAs have been investigated for various surface preparation procedures. In particular, in situ hydrogen plasma treatments were used to remove residual surface contamination (mainly O and C) and a nitrogen plasma to grow a thin mixed nitride layer. Al and Ti/Pt Schottky diodes with an ideality factor very close to 1, but with reduced barrier height, were found after the H2 plasma as a consequence of H diffusion into GaAs. The Schottky barrier height was further reduced if a H2 + N2 plasma was performed. The N content in the sputtering environment during the WNx deposition affects the diode properties of plasma-treated WNx contacts. By increasing the N2 partial pressure, the diode barrier height is reduced, probably due to nitridization of the GaAs surface. Such differences disappear after annealing the diodes in arsine overpressure at 800°C. WNx contacts deposited under different conditions on H2 plasma treated substrates also show a similar Schottky barrier height after such annealing. © 1991.