Atomic layer passivation (ALP) with atomically thin P-related compound semiconductor materials grown on (100) GaAs surfaces significantly enhances the photoluminescence intensity and also makes the electrical characteristics of GaAs metal-insulator-semiconductor (MIS) diodes (M/I/ALP/GaAs) nearly ideal. This study clarifies the change in GaAs interface characteristics caused by introducing As and subsequent passivation. Arsenic is introduced to GaAs surfaces by heating wafers in an AsH3, atmosphere before forming the passivation layer. Due to the As introduction, the enhancement of the PL intensity is suppressed to the magnitude of the intensity from conventional GaAs surfaces. The influence of As introduction on surface Fermi level pinning was also studied by fabricating MIS diodes. It is found that the range of capacitance becomes narrower than that observed in MIS diodes with ALP without an As layer. High-frequency analyses show that the surface Fermi level can move only within a limited energy range of 0.6 eV from the conduction band minimum of GaAs. It is shown that a reduction in the amount of As at the interface is necessary to unpin it in the ALP/GaAs system. The characteristics of As-induced interface states are discussed.