We have realized a 0.9nm-EOT TaSix/HfSiON gate stack that exhibits the high electron mobility of 264 cm(2)/Vs @ 0.8MV/cm (86% of thermal SiO2), even after spike annealing at 1000 degrees C. This was achieved by using thermally-stable HfSiON gate dielectrics with plasma nitridation, in which interfacial layer growth due to recoiled oxygen had been successfully suppressed.