A detailed study on PBTI relaxation after AC and DC stress in high-k nFETs is reported. First, Vt shift during AC and DC stress are examined, showing that the PBTI time evolution depends on the stress mode due to the relaxation effect. Then, comparison of relaxation after different stress types reveals large difference in the relaxation behavior at short times, whereas AC and DC relaxation are observed to merge at longer times. The "time-to-merge" rapidly increases with stress time and it strongly depends on the duty cycle. From a series of "Stress-Relax-Stress" measurement, we also demonstrate that the charge trapping and de-trapping process are highly correlated through "trap level". A simple model from a trap distribution point of view is proposed to rationalize the above observations. These observations provide new insight into the trapping dynamics during PBTI. © 2010 IEEE.