Separation of effects of statistical impurity number fluctuations and position distribution on Vth fluctuations in scaled MOSFETs

We have investigated the effect of the statistical "position" distribution of dopant atoms on threshold voltage (V/sub th/) fluctuations in scaled MOSFETs. The effects of impurity "number" fluctuations and impurity "position" distribution are successfully separated in two-dimensional simulation for fully-depleted (FD) SOI MOSFETs. It is found that the contribution by the position distribution is closely related to the charge sharing factor (CSF) and the effect of the impurity position distribution becomes dominant as CSF is degraded. Consequently, the contribution ratio of the impurity position distribution is kept almost constant when the device is properly scaled.