What effects would take place by doping nitrogen into high-k gate dielectric like SiON? In this study, nitrogen incorporation in Hf-based gate dielectric (HfO2 and HfSiO) has been studied. Thermal nitridation of Si prior to HfO2 deposition is one of the methods for incorporating N. However, it resulted in degraded interface. Thus, top nitridation was explored to prevent oxygen and boron penetration into Si substrate while maintaining high-k/Si interface. As a result, thermal stability and immunity to boron diffusion were improved. In addition, 2 times higher drive current was further enhanced by applying high temperature forming gas annealing at 600C. To achieve higher nitrogen concentration at the top, HfSiON (k 12-16) was used. None of nitrogen in the upper part of the dielectric diffused to the interface of HfO2 and Si substrate for anneals up to 800C. Even higher nitrogen concentration at the top was achieved by NH3 annealing of the gate dielectric which resulted in EOT