Diffusion is the movement of impurity atoms in silicon (or another semiconductor) at high temperature. Basically, diffusion can be described as a "smoothing out" of any localized high concentration of impurity atoms that is driven by the motion of the atoms in the material and by their mobility through the dominant atomic matrix (Figure 1). The impurity concentration gradient, dC/dx, is the driving force for diffusion. Diffusion can be mathematically expressed using Fick's first and second laws of diffusion. The diffusion of dopant atoms in silicon is reasonably predictable using these laws along with information on the temperature and time at temperature for a substrate. We will not discuss the quantitative relationships for diffusion here. For more details, see the excellent discussion of this topic in Wolf and Tauber's text and references therein.