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The work function is an important solid-state material parameter. It is defined as the energy difference between the Fermi level and the free-electron energy in a solid. The work function is a bit similar to the chemistry concept of ionization potential. In a metal, the Fermi level is located inside a partially-filled energy band, and the highest-energy electrons have energies close to the Fermi level. This suggests that the work function of a metal has the same intuitive meaning as ionization potential--it is the energy required to extract one electron from the metal. However, the Fermi levels of semiconductors and insulators are inside the forbidden energy band gap, so the ionization potential analogy is not suitable.

The work function naturally characterizes metallic phenomena such as thermionic emission (the basis for cathode ray tubes) and Einstein's photoelectric effect. In semiconductor junctions such as the p-n junction, work function differences directly correspond to potential energy barriers and curved energy bands.

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