PEPPER is a computer program that simulates in one dimension the ion implantation, diffusion, oxidation, epitaxy, deposition, and etch processes used in VLSI technology. The program contains an efficient Monte Carlo ion implantation algorithm that includes explicit calculation of ion channeling and damage. The key feature of the diffusion calculation is a general partial differential equation solver for rapid prototyping of physical models. The solver has been used to develop several unique diffusion models. A novel model for impurity diffusion in polysilicon treats the problem as a two-stream process, with relatively slow standard diffusion within the grain and a much more rapid component of diffusion along the grain boundaries. The two components are coupled at each point by a segregation term. Two other models for impurity diffusion in silicon include explicit calculation of the coupling of point defects with impurities. One of the point-defect models is a general and detailed formulation from a chemical kinetics viewpoint, while the other makes further assumptions to simplify the model for engineering analysis.>