TaN metal-gate nMOSFETs using HfTaO gate dielectrics have been investigated for the first time. Compared to pure HfO2, a reduction of one order of magnitude in interface state density (Dit) was observed in HfTaO film. This may be attributed to a high atomic percentage of Si-O bonds in the interfacial layer between HfTaO and Si. It also suggests a chemical similarity of the HfTaO-Si interface to the high-quality SiO2-Si interface. In addition, a charge trapping-induced threshold voltage (Vth) shift in HfTaO film with constant voltage stress was 20 times lower than that of HfO2. This indicates that the HfTaO film has fewer charged traps compared to HfO2 film. The electron mobility in nMOSFETs with HfO2 gate dielectric was significantly enhanced by incorporating Ta.