For a given emitter, with a given field applied to it, j F is independent of F, so eq. (21) shows that the shape of the distribution (as ε increases from a negative value well below the Fermi level) is a rising exponential, multiplied by the FD distribution function. This generates the familiar distribution shape first predicted by Young. At low temperatures, f F D ( ϵ ϵ ) goes sharply from 1 to 0 in the vicinity of the Fermi level, and the FWHM of the distribution is given by: