Abstract In this paper, recent results of Weibull slopes, area scaling factors, and breakdown behaviors observed for both soft breakdown and hard breakdown are discussed. These results would help to shed light on the breakdown mechanism of HfO 2 gate dielectrics. The Weibull slope β of the hard breakdown for both the area dependence and the time-to-dielectric-breakdown distribution was found to be β =2, whereas that of the soft breakdown was about 1.4 (EOT=14 A). We also integrated the time-to-breakdown characteristics of HfO 2 under unipolar AC voltage stress on MOS capacitors. The results show that longer lifetime of HfO 2 has been observed when compared to constant voltage stress. Higher frequency and lower duty cycle in the AC stress resulted in longer lifetime. As thickness decreases, the amount of lifetime enhancement decreases. The enhancement of unipolar t BD is attributed to less charge trapping during the “on time”, t on and charge detrapping during the off time, t off . It is proposed that time ( τ in ) for charge to be trapped in HfO 2 is longer than t on of unipolar stress under high frequency. In addition to experimental results, possible solutions are discussed.