The ZrTiON gate-dielectric GaAs metal-oxide-semiconductor (MOS) capacitors with or without ZrAlON as the interfacial passivation layer (IPL) are fabricated and their properties are investigated.The experimental results show that the GaAs MOS capacitor with the ZrAlON IPL exhibits better interfacial and electrical properties,including lower interface-state densitv (1.14 × 1012 cm-2eV-1),smaller gate leakage current (6.82 × 10-5 A /cm2 at Vfb+1 V),smaller capacitance equivalent thickness (1.5 am),and larger k value (26).The involved mechanisms lie in the fact that the ZrAlON IPL can effectively block the diffusion of Ti and O towards the GaAs surface,thus suppressing the formation of interfacial Ga-/As-oxides and As-As dimers,which leads to improved interfacial and electrical properties for the devices.