HfAlO/InAlAs metal–oxide–semiconductor capacitor (MOS capacitor) is considered as the most popular candidate of the isolated gate of InAs/AlSb high electron mobility transistor (HEMT). In order to improve the performance of the HfAlO/InAlAs MOS-capacitor, samples are annealed at different temperatures for investigating the HfAlO/InAlAs interfacial characyeristics and the device’s electrical characteristics. We find that as annealing temperature increases from 280 °C to 480 °C, the surface roughness on the oxide layer is improved. A maximum equivalent dielectric constant of 8.47, a minimum equivalent oxide thickness of 5.53 nm, and a small threshold voltage of –1.05 V are detected when being annealed at 380 °C; furthermore, a low interfacial state density is yielded at 380 °C, and this can effectively reduce the device leakage current density to a significantly low value of 1 × 10-7 A/cm2 at 3-V bias voltage. Therefore, we hold that 380 °C is the best compromised annealing temperature to ensure that the device performance is improved effectively. This study provides a reliable conceptual basis for preparing and applying HfAlO/InAlAs MOS-capacitor as the isolated gate on InAs/AlSb HEMT devices.