Thermionic emission

Thermionic emission (archaically known as the Edison effect) is the flow of electrons from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. The effect increases dramatically with increasing temperature (1000-3000 K), but is always present at temperatures above absolute zero. The science dealing with this phenomenon is thermionics. The charged particles are called thermions.

Richardson's Law

In any metal, there are one or two electrons per atom that are free to move from atom to atom. This is sometimes referred to as a "sea of electrons". Their velocities follow a statistical distribution, rather than being uniform, and occasionally an electron will have enough velocity to exit the metal without being pulled back in. The minimum amount of energy needed for an electron to leave the surface is called the work function, and varies from metal to metal. A thin oxide coating is often applied to metal surfaces in vacuum tubes to give a lower work function, as it is easier for electrons to leave the surface of the oxide.

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Richardson's Law, also called the Richardson-Dushmann equation, first published by Owen Willans Richardson in 1901, states that the emitted current density J (A/m2) is related to temperature T by the equation:

Related Topics:
Owen Willans Richardson - Current density - Temperature

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:J = A T^2 e^{-W over k T}

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where T is the metal temperature in kelvins, W is the work function of the metal, k is the Boltzmann constant. The proportionality constant A, known as Richardson's constant, given by

Related Topics:
Kelvin - Work function - Boltzmann constant

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:A = {4 pi m k^2 e over h^3} = 1.20173 imes 10^6 {A over m^2 K^2}

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where m and e are the mass and charge of an electron, and h is Planck's constant.

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The exponent in the equation shows that the amount of current emitted increases drastically with temperature.

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The thermionic emission equations are important in semiconductor design.

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