Impact of fixed charge at MOSFETs’ SiO2/Si interface on Vth variation

Randomness of discrete fixed charges at SiO₂/Si interface of NMOS, which is thought to be one of the possible origins of threshold voltage (V_th) variation, is investigated using 3D device simulation. Three cases of fixed charge types are assumed; (i) both negative and positive sheet charges exist with zero net charge (mix charges), (ii) only negative sheet charges exist, and (iii) only positive sheet charges exist. B_VT is used as a V_th variation indicator. It is found that, even if high concentration of fixed charge (10¹² cm^-2) is assumed, the difference of V_th variation between measured NMOS and random dopant fluctuation model by 3D TCAD still can not be explained, which reveals that other fluctuated parameters exists.