Types of electron emission

Types of electron emission

Thermionic emission: Electron emission takes place when a metal is heated to sufficient temperatures. This is known as thermionic emission and is employed in vacuum tubes. Thermionic emission at a temperature, T is given by the equation:-

I th = SA_oT²e ^{- E}_w^{/ kT} (1.14)

Where I th = Thermionic emission current in amperes.

S = Area of filament in m²

A_o = A constant

K = Boltzmann Constant, eV/^oK

T = Absolute temperature, ^oK

Ew= Work function, eV

Table 4.0 Properties of commonly used thermionic emitters and their applications

Field emission: When the potential difference between two electrodes in extremely high, electrons are emitted from the negative electrode, even at ordinary temperatures. The electric field at the surface of the negative electrode shall be in the range of 10 ⁶ volts / metre. This process of electron emission is called field emission. This type of emission is used in cold cathode devices and mercury and rectifier tube.

Photoelectric emission: Emission of electrons taken place when the surface of certain metals are impinged by a beam of light. Such an electron emission is called photoelectric emission and is used in photoelectric cells. The greater the intensity of light beam, the greater is the photoelectric emission.

Secondary emission: Free electrons are ejected when a beam of high velocity electrons strike a metal surface. This process is known as secondary emission.