ELECTRONICS | Thermionic Emission and Vacuum Tubes

Thermionic Emission and Vacuum Tubes

Thermionic emission is the assemblies of electrons from an electrode by virtue of its temperature (releasing of energy that is supplied by heat). This happens because the thermal energy given to a charge carrier overcomes the work function of the metal. The charge carriers may be electrons or ions, and according to older literature they are sometimes referred to as thermions. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralised by charge supplied by the battery as the emitted charge carriers rush away from the emitter, and finally the emitter will be in the same state like it was before emission.

A vacuum tube, electron tube, valve , or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

The simplest vacuum tube, the diode, introduced in 1904 by John Ambrose Fleming, contains only a heated electron- emitting cathode and an anode. Electrons can only flow in one specific direction through the device —from the cathode to the anode. Adding one or more control grids within the tube that allows the current flow between the cathode and anode to be controlled by the voltage on the grids.

Write the different modes of electron emission

There are four processes of electron emission as follows –

i) Thermionic emission – The energy required for electron emission from a metal is supplied in the form of thermal energy.

ii) Photo emission – When the electromagnetic radiation of frequency greater than threshold frequency is incident on a metal surface, electrons may be emitted from the metal.

iii) Field emission – If the energy required for electron emission is supplied in the form of electrostatic energy.

iv) Secondary emission – When the energy required for electron emission is supplied in the form of sufficient Kinetic energy of a moving particle like electron on ion.

Define work function

Work function

The minimum energy required to an electron to escape from the metal surface is known as work function of that metal.

* If ϕ be the work function in eV, then energy required by an electron to escape from metal surface = eϕ Joules.

*The maximum energy possessed by an electron at 0 K is called Fermi – energy (EF). All the energy states

below EF are filled up and all the states above EF are empty.

If E0 be the height of the surface potential energy barrier, then minimum energy required at 0 K that enables an electron to escape from the metal, is called work function of the metal i.e.

State Richardson’s Thermionic equation

Richardson’s thermionic equation

When a metal surface is heated, then the energy of the electrons inside the metal increases. When this energy is sufficient to overcome the potential energy barrier of the metal surface, then it is emitted from the metal surface.

Again work function

Obviously there is no possibility of electron emission at 0 K. If the temperature of the metal is increased then some of electrons may be excited to higher states having energy greater than E0 and escape from the metal.

Richardson and Dushman was derived a relation thermodynamically emission emission current density at a particular temperature ( Kelvin scale) of the metal as –

known as Riehardson – Dushman equation.

Where A => is a constant independent of the metal.

T=> is the temperature of the metal in Kelvin.

eϕ => is the work function of the metal.

k => is the Boltzmann constant.

Again

e = 1.6 × 10^-19 Coulomb.

k = 1.38 × 10^- 23 J/K

so, eϕ/k = b = 11600ϕ – is called temperature equivalent of work function. So

Explain the importance of Richardson’s equation in the field of Thermionic emission

Importance

1. The electron emission is markedly effected by the temperature change. Doubling the temperature may increase in electron emission more than 10^7 times.

2. Small change in work function (for different metal), can produce enormous increase in electron emission.

Calculate the work function in eV for sodium metal. Given, the threshold wavelength is 6800 Å.

Solution :

If ϕ be the work function then amount of energy need for emission of electron from metal eϕ Joules.

If ν0 be the threshold frequency,

Schottky effect

Schottky effect, increase in the discharge of electrons from the surface of a heated material by applying an electric field that reduces the value of the energy required for the electron emission.

For a fixed temperature of a cathode the plate current in a diode valve saturates for high plate voltages.This corresponds to total emission current of the cathode. Practically, it is found that the current does not truly saturate. It continues to increase slowly with the increase of plate voltage. Schottky gave an explanation of it and hence it is known as Schottky effect. The effect is named after its discoverer, the German physicist Walter H Schottky.