A study on the impacts of varying base oxide thickness, Si composition and nitridation on HfSiO to the overall high-k gate stack performance was carried out in details. Increasing base oxide thickness from 8A to 12A was found to reduce susceptibility of charge trapping within HfSiO layer and improve drive current. Also, increasing Si composition in HfSiO layer from 50% to 75% produced a higher drive current. However, this improvement was achieved at the expense of a higher gate leakage current. The HfSiO, when subjected to N2 plasma, forms HfSiON that exhibits excellent high-k dielectric properties with low EOT, low leakage current and high driving current . With complete understanding on the contribution from each layer, a good high-k gate stack, based on HfSiON was fabricated. Leakage current was successfully reduced to three orders lower than the conventional SiO2.