New insights into threshold voltage shifts for ultrathin gate oxides [MOSFETs]

Threshold voltage (V_th) shifts of p- and n-channel MOSFETs during stress are analyzed from both experiments and simulation. The result of the analysis showed that the 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 the proposed oxide degradation mechanisms, based on V_th shifts measured using I_d-V_g, may not be accurate.