What do you mean by electron emission ? Explain briefly various methods of electron emission.

Explanation

Electron emission :
Free electrons posses some kinetic energy and this energy is different for different electrons. The kinetic energy of the free electrons is not sufficient to overcome the surface barrier.
Whenever an additional energy is given to the free electrons, they will have sufficient energy to cross the surface barrier. And they escape from the metallic surface.
The liberation of electrons from any surface of a substance is called electron emission.
There are mainly four types of electron emission which are given below.
(i) Thermionic emission :
When a metal is heated to a high temperature, the free electrons on the surface of the metal get sufficient energy in the form of thermal energy so that they are emitted from the metallic surface. This type of emission is known as thermionic emission.

The intesity of the thermionic emission (the number of electrons emitted) depends on the metal used and its temperature.

Examples : cathode ray tubes, electron microscopes, X-ray tubes etc.
(ii) Field emission :
Electric field emission occurs when a very strong electric field is applied across the metal. This strong field pulls the free electrons and helps them to overcome the surface barrier of the metal.

Examples : Field emission scanning electron microscopes, Field-emission display etc.
(iii) Photo electric emission :
When an electromagnetic radiation of suitable frequency is incident on the surface of the metal, the energy is trasferred from the radiation to the free electrons. Hence, the free electrons get sufficient energy to cross the surface barrier and the photo electric emission takes place. The number of electrons emitted depends on the intensity of the incident radiation.

Examples : Photo diodes, photo electric cells etc.
(iv) Secondary emission :
When a beam of fast moving electrons strikes the surface of the metal, the kinetic energy of the striking electrons is transferred to the free electrons on the metal surface. Thus the free electrons get sufficient kinetic energy so that the secondary emission of electron occurs.

Examples : Image intensifiers, photo multiplier tubes etc.