Two-dimensional dopant mapping using secondary electrons in a scanning electron microscope is a useful and rapid technique for studying dopant distributions with high spatial resolution in semiconductor materials and devices. However, it has not yet found widespread application because the quantification of dopant concentrations currently lacks a firm theoretical model. This paper addresses the issue by means of Monte Carlo modelling. We demonstrate that by taking account of the electron affinity in Monte Carlo simulations to calculate the secondary-electron emission from doped silicon the dopant contrast can be explained. This paper also provides a firm theoretical model about the existence of surface effects in dopant contrast imaging.