Nonplanar VLSI Device Analysis Using the Solution of Poisson´s Equation

Techniques are presented for calculating the drain current of small-geometry MOSFET´s in the linear, subthreshold, and punch-through regions of device operation. The current calculation depends only on the electrostatic solution of the two-dimensional Poisson equation in the device. The accuracy of the techniques is established by comparisons with full two-dimensional simulations based on the simultaneous solution of the Poisson and minority-carrier current-continuity equations. The results of simulation also agree well with measurements on MOSFET´s having submicrometer effective channel lengths. The application of the simulations to nonplanar technologies is illustrated by the analysis of a taper-isolated dynamic-gain RAM cell. A description is given of simple numerical techniques for solving Poisson´s equation in the presence of nonplanar boundaries. The solution method demonstrates good convergence characteristics and minimizes computer storage requirements. Consequently, the simulation capabilities have been successfully implemented on a desktop calculator (Hewlett-Packard 9845) and on minicomputers (Hewlett-Packard 2100 and 1000-F).