We have investigated the effect of interface formation and processing conditions of Al2 O3 on GaAs on the density and distribution of interface state charge in the band gap. We have formed the insulator/semiconductor interface using both atomic layer deposition (ALD) and chemical vapor deposition (CVD). In situ ALD, ex situ ALD, and in situ CVD of aluminum oxide (Al2 O3) on GaAs were employed using metal-organic CVD. Isopropanol (IPA) was chosen as the oxygen source for Al 2 O3 deposition. No arsenic or gallium oxide was detected at the in situ ALD Al2 O3 /GaAs interface, while gallium oxide was observed at the in situ CVD Al2 O3 /GaAs interface. The entire distributions of interfacial defects from different processes were determined by conductance frequency method with temperature-variation capacitance-voltage (C-V) measurements. The existence of Ga2 O3 at the interface was found to be a possible method to lower the density of midgap defect states. From the C-V simulation, the midgap defect states are acceptorlike, which may originate from gallium vacancies near the interface. These states may also result in high frequency dispersion observed in the C-V curves of n-type metal-oxide-semiconductor field effect transistors. We correlate the interfacial defect states with the processes used to form the insulator-semiconductor interface. © 2011 American Institute of Physics.