Germanium is promising for future advanced CMOS devices because of its high mobility. But the lack of a stable Ge oxide makes it difficult to fabricate a germanium MOSFET. Inspired by the recent progress of high-k dielectric gate stack in Si MOSFET applications, the MOSFETs with Ge substrate have attracted more and more attention again. High-mobility p-channel germanium MOSFETs with ultra low EOT (6-10Å) have been reported with ultra-thin ZrOz.' Among various high-K gate dielectrics, HtO2 and Hf based dielectric materials have been researched as the most promising candidates for gate stack. Bai et al studied the Ge MOS capacitors with HfO2 dielecmcs.' It was found that the HfO2 without surface nitridation showed both a large EOT and a large leakage current density, while surface nitridation can suppress the interfacial growth and further achieve a small EOT and low leakage current at the same time. However, surface nitridation increases the process complexity and the presence of hydrogen increases the trapping rates. Hahium.oxynitride HfOx,NHfOy has been proved to be a high stable gate dielectric material with simple process procedure. It has higher crystallization temperature which may be beneficial to keep a low leakage current, and the nitrogen incorporation strengthens the immunity to oxygen diffusion which can suppress the interfacial layer growth for a low EOT. In this work, we studied the electrical performance of Ge MOSFET with a thin Hf0x,NHf0y gate dielectric.