Heating a metal gives some Electrons enough energy to escape its surface. A heated diode cathode can therefore supply an electric current across a vacuum gap to a collecting diode anode. This makes temperature a control for the supply of Electrons. Conversely, measuring that supply at several temperatures lets us estimate the surface work function: the energy barrier measured from the Fermi level to the nearby vacuum level. A Richardson plot turns the strong exponential function dependence into a slope.
Three questions must be kept separate: how many Electrons cross the surface, how many traverse the gap, and how many the collecting diode anode records. The Richardson-Dushman law answers the first under specified surface assumptions. Space charge can obstruct the second, while incomplete collection can alter the third.
Thermionic emission is a phenomenon in which electrons are emitted from a material, typically a metal or semiconductor, when it is heated to a high temperature. This occurs because heating the material provides sufficient energy to overcome the work function, which is the minimum energy required for an electron to escape from the surface of the material. In thermionic emission, as the temperature of the material increases, more electrons acquire enough thermal energy to break free from the attractive potential of the atomic lattice.
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