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A systematic study of mobility performances and Bias Temperature Instability (BTI) reliability was done on a large variety of advanced dielectric stacks. We clearly demonstrate that mobility performances and NBTI reliability are strongly correlated and that they are affected by the diffusion of nitrogen species N at the Si interface. Reducing the metal gate thickness favors the reduction of mobility degradations and NBTI, but, also strongly enhances PBTI, due to a complex set of reactions in the gate oxide. An optimum gate thickness must be found to obtain an acceptable trade off between device performance and reliability requirements.