We investigated ZnO surface passivation of a GaAs (100) substrate using an atomic layer deposition (ALD) process to prepare an ultrathin ZnO layer prior to ALD-HfO2 gate dielectric deposition. Significant suppression of both Ga-O bond formation near the interface and As segregation at the interface was achieved. In addition, this method effectively suppressed the trapping of carriers in oxide defects with energies near the valence band edge of GaAs. According to electrical analyses of the interface state response on p- and n-type GaAs substrates, the interface states in the bottom half of the GaAs band gap were largely removed. However, the interface trap response in the top half of the band gap increased somewhat for the ZnO-passivated surface.