MODELING STATISTICAL DOPANT FLUCTUATIONS IN MOS-TRANSISTORS

The impact of statistical dopant fluctuations on the threshold voltage ii and de,ice performance of silicon MOSFET's is investigated by mean s of analytical and numerical modeling. A new analytical model describ ing dopant fluctuations in the active device area enables the derivati on of the standard deviation, sigma(VT), of the threshold voltage dist ribution for arbitrary channel doping profiles. Using the MINIMOS devi ce simulator to extend the analytical approach, it is found that sigma (VT) can be properly derived from two-dimensional (2-D) or three-dimen sional (3-D) simulations using a relatively coarse simulation grid, Ev aluating the threshold voltage shift arising from dopant fluctuations, on the other hand, calls for full 3-D simulations with a numerical gr id that is sufficiently refined to represent the discrete nature of th e dopant distribution. The average V-T-shift is found to be positive f or long, narrow devices, and negative for short, wide devices. The fas t 2-D MINIMOS modeling of dopant fluctuations enables an extensive sta tistical analysis of the intrinsic spreading in a large set of compact model parameters for state-of-the-art CMOS technology. It is predicte d that V-T-variations due to dopant fluctuations become unacceptably l arge in CMOS generations of 0.18 mu m and beyond when the present scal ing scenarios are pursued. Parameter variations can be drastically red uced by using alternative device designs with ground-plane channel pro files.