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The impact of AlGaAs and AlAs buffer layers on the electrical properties of an epitaxial gallium-arsenide (epi-GaAs) metal-oxide-semiconductor capacitor (MOSC) was investigated. MOSC was fabricated by using atomic-layer-deposited Al2O3 -TiO2 (TiAlO) alloy gate dielectric and epi-GaAs layers. The epi-GaAs layer was grown on Ge substrates at 675 °C with and without buffer layer between epi-GaAs layer and Ge substrates. The TiAlO/epi-GaAs interface with an AlGaAs buffer layer allows realizing a high-quality interface between epi-GaAs layers and TiAlO dielectric, much sought after for high-speed transistor applications on a silicon platform. TiAlO dielectric is amorphous even upon annealing at 500 °C and exhibits a sharp interface with epi-GaAs layers. The choice of AlGaAs over AlAs for a buffer layer was made based on the quality of resulting TiAlO/epi-GaAs surface passivation as evident through structural and electrical characteristics. Epi-GaAs with an AlGaAs buffer layer was found to improve the performance of the MOSC significantly through increase in accumulation capacitance and breakdown voltage. The interface state density, flatband voltage, frequency dispersion, and leakage current were decreased for the MOSC fabricated with an AlGaAs buffer layer.