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In this letter, we present a comprehensive study on longterm reliability of ultrathin TaN-gated chemical vapor deposition gate stack with EOT=8.5-10.5. It is found that, due to the asymmetric band structure of HfO2 gate stack with an interfacial layer, the HfO2 gate stack shows polarity-dependent leakage current, critical defect density, and defect generation rate, under gate and substrate injection. However, no such polarity dependence of time-to-breakdown (TBD) is observed when TBD is plotted as a function of gate voltage. The 10-year lifetime of an HfO2 gate stack is projected to be Vg=-1.63 V for the equivalent oxide thickness (EOT) =8.6 and Vg=-1.88 V for EOT=10.6 at 25°C. These excellent reliability characteristics are attributed to reduced leakage current of HfO2 gate stack with physically thicker films that result in larger critical defect density and Weibull slope to that of SiO2 for the same EOT. However, at 150°C, and with area scaling to 0.1 cm2 and low percentile of 0.01%, the maximum allowed voltages are projected to Vg=-0.6 V and -0.75 V for EOT of 8.6, and 10.6, respectively.