The composition of the SiON film were optimized to be used as an anneal encapsulation based on the GaAs metal-semiconductor field effect transistor (MESFET) characteristics. The SiON films investigated in the present study were deposited by plasma-enhanced chemical vapor deposition. The refractive indexes of the SiON films with various compositions were found to be useful indicators of film composition. By varying the ratio of the source gases, SiH4 and N2O, we could control the composition to range from SiO2 to Si3N4 corresponding to refractive indexes from 1.47 to 2.02. MESFET characteristics were evaluated in terms of the Schottky diode ideality factor n, which was found to be highly dependent on the film composition : when we used a SiON film with a refractive index below 1.53, which corresponds to an O-rich composition, the n value exceeded 1.4. This indicates an unstable interface between the GaAs and the SiON. When we used the N-rich SiON, however, such as SiON with a refractive index 1.6, the n value was 1.25. These results are discussed from the viewpoint of interface reaction between GaAs and SiON, which we investigated by using Auger electron spectroscopy.