Charged defects in the gate insulating oxide cause various degradations in operating MOSFET such as flat-band voltage shifts or threshold voltage instabilities. Excess or deficient oxygen atoms in the oxide layer are relevant to those fixed charges, and thus oxygen concentration must be carefully controlled in the fabrication process. Given the fact that the optimum ambient atmosphere strongly depends on the size or structure of the gate stack, adjusting oxygen partial pressure is not realistic. Our computational and thermodynamic investigations suggest that multivalent oxides such as Ce-oxide act as an oxygen reservoir when deposited on a high-k oxide, and play an important role in retaining the oxygen chemical potential unchanged throughout the oxide film independent of the process conditions. Moreover, we demonstrate that finding the best combination of the multivalent oxide with a high-k oxide can minimize the number of charged defects. This theoretical model has been justified by our experimental observations for Ce-oxide/La2O 3 gate oxide in comparison with La2O3.