We have studied the effects of nitrogen concentration ([N]=N/(Hf+Si+O+N)) in HfSiON films and post-nitridation annealing (PNA) conditions on the reliability of metal oxide semiconductor field effect transistors (MOSFETs) with HfSiON gate dielectrics in inversion states with constant voltage stress. In nMOS case, higher [N] and lower PNA temperatures are effective to reduce stress induced leakage current (SILC). SILC is related to crystallinity in HfSiON films. On the other hand, only small SILCs were observed in pMOSs, and the currents were independent of [N] and PNA temperatures without regard to crystallinity in the HfSiON films. Differences in SILC behavior between nMOS and pMOS are related to the electron transport mechanism. It is thought that SILC was generated by increasing positive oxygen vacancies whose energy level is near the conduction band edge of HfSiON. Since electrons in pMOS move through the deep trap level, their transit is independent of positive oxygen vacancies. For highly reliable HfSiON gate films, it is important to form homogeneous and amorphous HfSiON films.