The study explored titanium dioxide (TiO 2 ) prepared by liquid phase deposition (LPD) deposited on (NH 4 ) 2 S x -treated aluminum gallium arsenide (AlGaAs), including the surface roughness by using atomic force microscopy (AFM) measurements and electrical properties. The leakage current density of MOS capacitor is approximately 6.83 × 10 −7 A/cm 2 at zero electric field for the sample without any pretreatment. The interface trap density (D it ) and the flat-band voltage shift (ΔV FB ) are 4.46 × 10 12 cm −2 eV −1 and 3.6 V, respectively. After the 10 min 5% (NH 4 ) 2 S x pretreatment, the leakage current density, D it , and ΔV FB can be improved to 1.04 × 10 −7 A/cm 2 at zero electric field, 2.28 × 10 12 cm −2 eV −1 , and 2 V, respectively. The paper also demonstrates the application to the AlGaAs/InGaAs metal-oxide-semiconductor pseudomorphic high-electron-mobility transistor (MOS-PHEMT) using LPD-TiO 2 after 10 min sulfide pretreatment.