The GaAs MOS capacitor was fabricated with HfTiON as high-k gate dielectric and NH_3-plasma-treated ZnON as interfacial passivation layer(IPL), and its interfacial and electrical properties are investigated compared to its counterparts with ZnON IPL but no NH_3-plasma treatment and without ZnON IPL and no plasma treatment. Experimental results show that low interface-state density near midgap(1.17 ×10~(12) cm ~(-2)e V~(-1)/and small gate leakage current density have been achieved for the Ga As MOS device with the stacked gate dielectric of HfTi ON/ZnON plus NH_3-plasma treatment. These improvements could be ascribed to the fact that the ZnON IPL can effectively block in-diffusion of oxygen atoms and out-diffusion of Ga and As atoms, and the NH_3-plasma treatment can provide not only N atoms but also H atoms and NH radicals, which is greatly beneficial to removal of defective Ga/As oxides and As-As band, giving a high-quality ZnON/GaAs interface. The GaAs MOS capacitor was fabricated with HfTiON as high-k gate dielectric and NH_3-plasma-treated ZnON as interfacial passivation layer (IPL), and its interfacial and electrical properties are investigated compared to its counterparts with ZnON IPL but no NH_3-plasma treatment and without ZnON IPL and no plasma treatment. Experimental results show that low interface-state density near midgap (1.17 × 10 ~ (12) cm ~ (-2) e V -1 / small gate leakage current density have been achieved for the Ga As MOS device with the stacked gate dielectric of HfTi ON / ZnON plus NH 3-plasma treatment. These improvements could be ascribed to the fact that the ZnON IPL can effectively block in-diffusion of oxygen atoms and out-diffusion of Ga and As atoms, and the NH 3-plasma treatment can provide only N atoms but also H atoms and NH radicals, which is greatly beneficial to removal of defective Ga / As oxides and As-As bands, giving a high-quality ZnON / GaAs interface.