Threshold voltage instability characteristics under positive dynamic stress in ultrathin HfO2 metal-oxide-semiconductor field-effect transistors

The characteristics of threshold voltage instability of HfO2 metal-oxide-semiconductor field-effect transistors under positive dynamic stress were investigated. Compared to constant stress, a smaller threshold voltage shift was observed at higher frequency and lower duty cycle under unipolar pulse stress. Similarly, the degradation of maximum transconductance was also reduced with unipolar stress conditions. However, the change of subthreshold swing was found to be negligible and fairly independent of stress frequencies and duty cycles. The traps in the bulk of the HfO2 dielectric are believed to be the primary factor for the larger change in threshold voltage. Compared to the result under constant voltage stress, unipolar stress allowed higher ten-year lifetime operating voltage.