What is Thermionic Emission : Working & Its Applications

The phenomenon of thermionic emission was first observed by Thomas A in 1883. He observed that electricity can supply from a filament to a metal plate within an incandescent lamp. In thermionic emission, electrons emission can be done from heated materials which are broadly used in conventional electron tubes as a source of electrons in the fields of electronics & communication. The best example of this emission is, electrons can emit from a hot cathode and enter into a vacuum within a vacuum tube. So, this article discusses an overview of thermionic emission, derivation, advantages & its applications.

What is Thermionic Emission?

Thermionic emission definition is, when the heat energy is applied to metal then it emits electrons from the surface of the metal and it is also known as the thermionic emission effect. The term ‘Thermionic’ can be formed from the two words namely Thermal (heat) & ions (charged particles). The thermionic emission diagram is shown below.

Thermionic Emission
Thermionic Emission

What Factors Affect Thermionic Emission?

There are three factors that affect this emission like metal surface temperature, metal surface area & the function of the metal.

The temperature of Metal Surface

When the metal surface temperature is high then the emission rate of electrons from the metal surface is higher.

Metal Surface Area


When the metal surface area is larger, then the rate of electrons emitted from the metal surface is high.

Function of Metal

The work function of the metal is low then the rate of electrons emission from the metal surface is high.

Thermionic Emission Working

Thermionic emission mainly occurs in metals when they are heated at very high temperatures. Metals generally work under two conditions metals in normal temperature and metals in high temperature which is discussed below.

Thermionic Emission in Metals
Thermionic Emission in Metals

Metals in Normal Temperature

Once the normal temperature is provided to the metal, then the valence electrons can gain sufficient energy & break the connection with the parent atom to become free, which is known as a free electron.
In the metal, the free electrons will have some kinetic energy (K.E) but they do not contain sufficient energy to get away from the metal. So, the attractive force from the atomic nucleus will resist the free electrons from the metal which tries to escape.

The energy of the free electrons within the metal is low as compared to the electrons within a vacuum. Thus, free electrons need additional energy from the external source to move into the vacuum.

Metals under High Temperature

Once the high temperature is applied to the metal, then free electrons will get sufficient energy & conquer the attractive force from the atomic nucleus, which holds the free electrons within the metal.
The free electrons in the metal overcome the attractive force of the atomic nuclei break the connection with the metal & move into the vacuum. This emission mainly occurs in metals when they are heated at very high temperatures.

Once the heat energy is supplied to the metal, then free electrons will escape from the metal surface which is known as thermions. This emission process plays a key role within the operation of the electronic device.

Difference b/w Thermionic Emission and Photoelectric Effect

The difference between thermionic emission and the photoelectric effect is discussed below.

Thermionic Emission Photoelectric Effect
In this emission, electrons are emitted from the metal surface by supplying heat energy. In photoelectric emission, light energy can be emitted once electrons from the metal surface are emitted.
The electrons which are emitted in this is known as thermions. The electrons which are emitted in this effect are known as photoelectrons.
In this phenomenon, ample thermal energy is introduced to the free electrons through a heating process. So they emit from the metal surface. In this phenomenon, electrons emission from the metal surface occurs when light energy drops on it.
The obtained energy by free electrons for their motion & emission comes from thermal sources. The energy toward emission electrons is being supplied through light photons.
This emission can be caused by thermal energy or heat. This emission can be caused by the electromagnetic energy of light.
The emission of electrons can be done at a specific temperature. Electrons are emitted at threshold frequency.
When temperature increases, the rate of time for electrons emission will be increased. The time rate of electrons emission enhances with the increase within intensity.

Thermionic Emission Equation Derivation

During the emission process, the emitted electrons mainly depend on the metal surface area as well as the temperature of the metal surface. So, it can be mathematically expressed with the help of O. W. Richardson Dushman’s equation.

This equation tells the emission’s current density to the temperature (T) & work function (W) of the emitting material. So, the thermionic emission formula is;

J = AT2exp(-W/kT)

From the above equation,

‘j’ is the current density of the electron emission (mA/mm^2)

‘T’ is surface temperature in Kelvin (K)

‘A’ is the Plank’s constant in Amperes/m2/k2

‘W’ is the cathode material’s work function in J or eV

‘k’ is the Boltzmann constant like 1.3806488E^-23 J K-1 or 8.6173324E-5 eV K-1

For most of the metals, the proportionality constant value ‘A’ is also called Plank’s Constant and its value is 600,000, but it can simply change when metal characteristics change. The following equation determines the ‘b’ value which depends on the specific metal features. So, for particular metals, it is constant, although it shows differences through varying temperatures.

B = ɸe/k’

From the above equation,

‘B’ is the Boltzmann constant = 1.38×10^-23 J/K

‘ɸ’ is the work function of metal within eV

‘e’ is the electron charge = 1.602×10^-19 coulomb.

Therefore from the above equation, we can conclude that the maximum emission can be achieved through two things like metals work function should be low otherwise the temperature of the surface of metal should be high. If we achieve these two conditions, a significant no. of electrons will go away from the valence band & jump into the vacuum.


The advantages of Thermionic Emission include the following.

  • It plays a key role in both basic physics & digital electronic technology.
  • This emission discovery allows physicists to generate electrons beams within a vacuum.
    Thermionic sources are not expensive, so one can easily operate in fewer vacuum conditions & offer better brightness especially for illumination of large-area than sources of field emission.


The applications of Thermionic Emission include the following.

  • It is used in different applications like high-frequency-based vacuum transistors used in electronics, power electronics, electron guns used in scientific instrumentation, x-ray generation & energy converters from solar energy & sources of high temperature.
  • It is used in diode valves, vacuum tubes, cathode ray tubes (CRT), electron microscopes, electron tubes, electrodynamics tethers, etc.

What is thermionic emission in semiconductors?

At the semiconductor surface, if the charge carriers contain enough energy, then they may be emitted thermionically into the vacuum, so in a way similar to the metal vacuum case.

What is the function of thermionic emission?

Thermionic emission is the electrons discharge from heated materials which is extensively used as an electrons source within conventional electron tubes in the electronics & communications fields.

How do you increase the thermionic emission rate?

When the metal surface area is higher, then the emission rate will be high because a huge surface area provides more space to emit electrons.

What does thermionic emission produce and where does it occur?

Thermionic emissions produce charge particles like electrons from the heated surface of a metal. This emission occurs within metals when they are heated at very high temperatures.

Thus, this is all about an overview of thermionic emission. This process is significant in the process of different electronic devices & can be used for generation of power or cooling. Here is a question for you, what are the disadvantages of thermionic emission?