contact potential difference
Contact potential difference
An electrostatic potential that exists between samples of two dissimilar electrically conductive materials (metals or semiconductors with different electron work functions) that have been brought into thermal equilibrium with each other, usually through a physical contact. Although normally measured between two surfaces which are not in contact, this potential is called the contact potential difference. Initially it is expected that mobile charge carriers (electrons or holes) will migrate from one sample to the other. If there is a net flow of electrons from material A to material B (see illustration), material B will become negatively charged and material A will become positively charged, assuming that they were originally neutral. This process is self limiting because a potential difference between the two samples will develop due to the charge separation and will grow to a value sufficient to stop further motion of the electrons from A to B.
In a metal or a semiconductor, the electrons are distributed in energy such that virtually all of them exist at or below a level called the Fermi level. When any combination of metals and semiconductors are put into equilibrium with one another, the Fermi levels in all will coincide. The contact potential difference between materials is that value necessary to raise or lower the potential energies of the electrons to produce a common Fermi level. Since they are then at the same energy, electrons in either material will have no net force on them, that is, no reason to travel to the other material. Because it causes no net force on the equilibrium distribution of electrons, contact potential difference cannot be directly measured with an ordinary voltmeter. Nevertheless, it profoundly affects the behavior of a number of electronic devices. See Free-electron theory of metals, Semiconductor, Work function (electronics)