In situ PEALD processes, including PEALD-AlN pre-gate and post-gate plasma gas treatments, have been studied as a promising passivation method to realize the high interfacial quality of high-k/III-V structures. The formation of excellent dielectric gate stack has been obtained on the HfO2/n, p-In0.53Ga0.47As MOSCAPs by inserting an AlN interfacial layer. The improvements on the electrical properties of HfO2/In0.53Ga0.47As nMOSFET such as maximum drain current, subthreshold swing, off leakage current, and effective electron mobility have been achieved. Utilizing low frequency noise, charge pumping, and bias temperature instability stress technologies, the reliability of the in situ PEALD-passivated HfO2/In0.53Ga0.47As nMOSFET is evaluated.