Threshold voltage instability characteristics of HfO2 dielectrics n-MOSFETs

When different polarity of stress, such as positive, negative, and bipolar stress was applied, it was observed that frequency and duty cycle dependencies were still valid in all three conditions. In contrast to positive stress, negative stress showed a decrease in the threshold voltage shift. Bipolar stress resulted in the highest threshold voltage instability, but the degradation in transconductance and subthreshold swing was actually smaller than those in negative unipolar stress. The bulk trap of HfO2 dielectric, which is proportional to its physical thickness, is believed to be the primary factor for threshold voltage shift. AC unipolar operation would allow a higher 10-year lifetime operating voltage than the DC condition. In addition to experimental results, a plausible mechanism has been proposed.