Threshold voltage instability of HfSiO dielectric MOSFET under pulsed stress

The effects of on-time, off-time, and rise/fall time of AC stress on V/sub th/ shift in HfSiO gate dielectric NMOSFETs have been studied for pulsed stress reliability testing of high-k devices. In slow rise/fall time conditions, charge trapping is only dependent on the sum of on-times. However, as the rise/fall time becomes shorter, a significantly increased charge trapping has been detected. This charging behavior becomes more effective at higher frequency. For the relaxation during off-time to be effective, a longer time than 10/sup - 1/ sec is required. As a result of the relaxation test, it is suggested that charge redistribution would occur during on-time due to the applied field. Therefore, the relaxation would be harder in the sample stressed longer because the distribution of charge traps is more stable.