New insights into threshold voltage shifts for ultrathin gate oxides

Threshold voltage (V_th) shifts of p- and n-channel MOSFETs during a stress are analyzed from both experiment and simulation. The result of the analysis showed that V_th shift is mainly induced by the carrier channel mobility degradation. This result can explain the polarity dependence of V_th shifts in p- and n-channel MOSFETs. Besides, it suggested that the commonly accepted idea that V_th shifts are due to Coulombic charge generation in the oxide affecting the surface potential is not accurate. It also suggested that proposed oxide degradation mechanisms based on V _th shifts measured using I_d-V_g may not be accurate.