Threshold voltage instability of ultra-thin HfO/sub 2/ NMOSFETs: characteristics of polarity dependences

Threshold voltage instability characteristics of high-k HfO/sub 2/ with SiON interface NMOSFETs under three different dynamic stress conditions, positive, negative, and bipolar stress, have been investigated for the first time. Frequency and duty cycle dependencies have been observed in all three conditions. In contrast to positive AC stress, negative dynamic stress showed decrease in the threshold voltage. Bipolar stress resulted in the highest threshold voltage shift, but the degradation in transconductance and subthreshold swing was actually smaller in comparison to those in negative unipolar stress. A plausible mechanism has been proposed.