The dependence of the electron mobility in n-type and p-type Si on the impurity concentration is studied using a simplified picture that, hopefully, retains the essential physics. The momentum relaxation time is calculated including interactions between electrons and phonons, electrons, and ionized impurities using a phase-shift analysis, short-range intercarrier interactions, and, most importantly, the scattering of electrons by plasmons. It is found that this process has a very strong effect at large impurity concentrations, particularly in p-type Si. Recent experimental results for the minority-electron mobility in the base of p-n-p bipolar transistors can be explained in terms of the proposed model, although only qualitative aspects can be considered, since the scatter of the experimental data is often larger than the differences caused by the various approximations employed. © 1991 The American Physical Society.