For SiO2 or SiON, negative bias temperature instability (NBTI) generally follows a power law. There is less information available for the NBTI of Hf stacks and it will be studied and compared with that of SiO2 in this work. We found that the power factor for Hf stacks was substantially smaller and the NBTI kinetics has a "flat-then-rise" feature. The flat region at short stress time originates from the preexisting cyclic positive charge in Hf stacks, which is different from the defect responsible for the rising part at longer time and leads to the smaller power factor for Hf stacks. (c) 2008 American Institute of Physics.