In this paper, we present the interfacial and electrical properties of GaAs MOS capacitors fabricated using RF-sputtered TaAlO x dielectrics with and without a silicon interface control layer (ICL). As-deposited films are analyzed by X-ray photoelectron spectroscopy, which reveals that the suppression of low-k (Ga-O and As-O) interfacial layer formation is achieved with the introduction of an ultra-thin Si ICL. Silicon-passivated GaAs MOS capacitors exhibit excellent electrical properties featuring low hysteresis of 0.2 V and minimum interface-trapped charge density D it of 3.1 × 1012 eV−1 cm−2. The TaAlO x /Si/n-GaAs stack shows very low-frequency dispersion (~2.8 %), which is attributed to the reduction of interfacial oxides (Ga-O and As-O) and thus unpins the Fermi level. The current density of 5.1 × 10−5 A cm−2 at 1 V with an EOT of 2.4 nm is achieved in GaAs MOS capacitors with the introduction of a Si ICL. Furthermore, the electrical conduction mechanisms of TaAlO x thin films are also investigated as a function of temperature. Our findings unveil that the introduction of a silicon inter-layer between TaAlO x and GaAs becomes an effective way to improve interfacial and electrical characteristics of the MOS capacitor.