The electrical characteristics of liquid-phase-deposited (LPD) silicon oxynitride film on ammonium-sulfide-treated p-type (100) gallium arsenide (GaAs) substrate were investigated. Hydrofluosilicic acid, ammonia, and boric acid aqueous solutions were used as precursors. The electrical characteristics of silicon oxynitride film are a function of the boric acid volume in the growth solution and much improved on GaAs substrate with ammonium-sulfide treatment. The leakage currents can reach 1.24 × 10−7 and 7.85 × 10−7 A cm−2 at ±0.5 MV cm−1. The lowest interface state density and the dielectric constant are 3.32 × 1011 cm−2 eV−1 and 4.74, respectively.