Simulation and two-dimensional analytical modeling of subthreshold slope in ultrathin-film SOI MOSFETs down to 0.1 mu m gate length

The subthreshold slope in ultra-thin-film fully depleted SOI MOSFETs is investigated for channel lengths from the long channel region down to 0.1 mu m. A doping effect is found which allows improvement of the S-factor by increasing the channel doping concentration. In order to explain this phenomenon and to clarify the mechanism of S-factor degradation at short gate lengths, a two-dimensional analytical model is developed. A modified boundary condition for the two-dimensional Poisson equation is introduced to account for the nonlinear potential distribution inside the buried oxide. It is found that the S-factor short-channel degradation is governed by three mechanisms: the rise of capacitances at the channel source and drain ends due to the two-dimensional potentional distribution; the subthreshold current flow at the back channel surface; and the modulation of the effective current channel thickness during the gate voltage swing in the subthreshold region. The analytical model results are compared to those of numerical device simulation, and a good agreement is found.<
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