SIMULATION AND 2-DIMENSIONAL ANALYTICAL MODELING OF SUBTHRESHOLD SLOPE IN ULTRATHIN-FILM SOI MOSFETS DOWN TO 0.1 MU-M GATE LENGTH

The subthreshold slope in ultrathin-film fully depleted SOI MOSFET's is investigated for channel lengths from the long channel region down to 0. 1 mum. A doping effect is found which allows us to improve 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 length, a two-dimensional analytical model is developed. A modified boundary condition is introduced for the two-dimensional Poisson equation 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 potential 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. The model can be utilized to predict design criteria for miniaturized thin-film fully depleted SOI MOSFET's.