A historical note on methods of deriving Fowler–Nordheim-type equations is necessary. There are several possible approaches to deriving these equations, using free-electron theory. The approach used here was introduced by Forbes in 2004 and may be described as "integrating via the total energy distribution, using the parallel kinetic energy K p as the first variable of integration". Basically, it is a free-electron equivalent of the Modinos procedure (in a more advanced quantum-mechanical treatment) of "integrating over the surface Brillouin zone". By contrast, the free-electron treatments of CFE by Young in 1959, Gadzuk and Plummer in 1973 and Modinos in 1984, also integrate via the total energy distribution, but use the normal energy ε n (or a related quantity) as the first variable of integration.