This work describes the preparation of a passivation AlOx layer grown in-situ in an MOVPE reactor immediately after the growth of a III-V semiconductor structure. Aluminum layers were deposited on GaAs surface at temperatures of 650, 600, 500, and 450 degrees C, and AlOx layers were formed spontaneously from the Al layers upon exposing the surface to the air outside the reactor. The best thickness uniformity of the AlOx layers was observed for a deposition temperature of 450 degrees C. Significantly higher drain current densities were achieved for high electron-mobility transistors (HEMTs) prepared on structures with the AlOx layers. We attribute this to a better surface stability of the structures during device processing.