Undoped and doped FeSi or how to make a heavy fermion metal with three of the most common elements

We review electrical conductivity, magnetic susceptibility and optical data on the Kondo insulator FeSi and its chemically doped metallic derivatives. While at T=0, FeSi appears in many respects to be a conventional band insulator, its temperature-dependent behavior cannot be explained in terms of ordinary band theory. Doping with Al yields a transition from the Kondo insulator to a heavy fermion metal. The transition maps quantitatively onto that observed for Si:P, with the exception of a strongly renormalized effective carrier mass and critical concentration. We conclude that one of the simplest ways to make a heavy fermion metal is to dope a Kondo insulator.