This study presents the fabrication and improved properties of an AlGaAs/InGaAs metal-oxide-semiconductor pseudomorphic high-electron-mobility transistor (MOS-PHEMT) using liquid phase deposited titanium dioxide (LPD-TiO2) as a gate dielectric. Sulfur pretreatment and postoxidation rapid thermal annealing (RTA) were consecutively employed before and after the gate dielectric was deposited to fill dangling bonds and therefore release interface trapped charges. Compared with a benchmark PHEMT, the AlGaAs/InGaAs MOS-PHEMT using LPD-TiO2 exhibited larger gate bias operation, higher breakdown voltage, suppressed subthreshold characteristics, and reduced flicker noise. As a result, the device with proposed process and using LPD-TiO2 as a gate dielectric is promising for high-speed applications that demand little noise at low frequencies.