Time-dependent dielectric breakdown and stress-induced leakage current (SILC) behavior of a 28-nm hafnium-based high-k/metal gate n-type MOS transistors are investigated under dc and ac conditions. Compared with the constant voltage stress results under dc conditions, a decrease in device lifetime is observed for low-frequency ac stress. Degradation enhancement and SILC behavior are attributed to defect generation in the SiO₂ interface layer. For high-frequency ac stress, a lifetime improvement is observed, because of reduced trap generation.