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Effects of a TiN gate electrode on interface trap density, bias temperature instability (BTI), and time-dependent dielectric breakdown (TDDB) in HfSiON metal-oxide-semiconductor field-effect transistors are investigated in this paper. Based on experimental data, we found that the TiN metal gate electrode thickness plays an important role in determining the final dielectric stability and the interface quality. Samples with thicker TiN gate electrode, which prevent oxygen diffusion from the high-κ layers toward the α-Si electrode, exhibit a lower Dit level. In addition, the levels of oxygen vacancies are expected to be suppressed by thicker TiN gate electrode, which subsequently alleviates damage at the Si/SiO2 interface and improves both the BTI and TDDB performances.