The Free-Electron Model

Abstract

The characteristic properties of metals and semiconductors are due to their conduction electrons: the electrons in the outermost atomic shells, which in the solid state are no longer bound to individual atoms, but are free to wander through the solid. A proper understanding of metallic or semiconducting behaviour could not begin, obviously, until the electron had been discovered by J.J. Thomson in 1897, but once this had happened, the significance of the discovery was at once recognized. By 1900 Drude had already produced an electron theory of electrical and thermal conduction in metals, which (with refinements by Lorentz a few years later) survived until 1928. Not surprisingly, this very early theory did not manage to explain everything — after all, the structure of the atom itself was quite unknown until Rutherford and his co-workers discovered the nucleus in 1911 — but it did have one or two striking successes, and it is worth starting with a brief look at this classical model, because it already contained many of the right ideas.