Modeling statistical dopant fluctuations in MOS transistors

Abstract

The impact of statistical dopant fluctuations on the threshold voltage V/sub T/ and device performance of silicon MOSFET's is investigated by means of analytical and numerical modeling. A new analytical model describing dopant fluctuations in the active device area enables the derivation of the standard deviation, /spl sigma/V/sub T/, of the threshold voltage distribution for arbitrary channel doping profiles. Using the MINIMOS device simulator to extend the analytical approach, it is found that /spl sigma/V/sub T/, can be properly derived from two-dimensional (2-D) or three-dimensional (3-D) simulations using a relatively coarse simulation grid. Evaluating the threshold voltage shift arising from dopant fluctuations, on the other hand, calls for full 3-D simulations with a numerical grid that is sufficiently refined to represent the discrete nature of the dopant distribution. The average V/sub T/-shift is found to be positive for long, narrow devices, and negative for short, wide devices. The fast 2-D MINIMOS modeling of dopant fluctuations enables an extensive statistical analysis of the intrinsic spreading in a large set of compact model parameters for state-of-the-art CMOS technology. It is predicted that V/sub T/-variations due to dopant fluctuations become unacceptably large in CMOS generations of 0.18 /spl mu/m and beyond when the present scaling scenarios are pursued. Parameter variations can be drastically reduced by using alternative device designs with ground-plane channel profiles.