What is Electron Emission?
The liberation of electrons from the surface of a substance is known as electron emission. For electron emission, metals are employed because they have many free electrons.
In a metal like copper, platinum or tungsten, there are plenty of free electrons. At room temperature these electrons are free only to the extent that they may transfer from one atom to another within the metal but they cannot leave the metal surface. A free electron cannot escape from the metal surface because it is pulled back by the positively charged atom. The force exerted at the surface of metal that prevents the free electrons from escaping, is called the surface barrier. The attractive surface forces tend to keep the electrons within the metal surface, except for a small portion that happens to have sufficient kinetic energy to breakthrough the energy barrier caused by these forces. The majority of electrons move too slowly for this to happen.
The electrons are required to perform a certain amount of work to overcome the restraining forces in order that they may escape out of the surface of a metal. To do this work the electrons must be imparted with sufficient energy from some external source of energy, since their own kinetic energy is inadequate. The amount of energy required for the escape of electrons varies with different materials i.e., electron emission can take place more easily in some materials than in others.
Modern physics tells us that even at the absolute zero temperature, the velocity (or kinetic energy) of all the electrons does not reduce to zero; there are many electrons that possess appreciable energy. The maximum energy that an electron in a metal has, at the absolute zero temperature, is called the Fermi level of energy and is designated as EF. If EB is the total barrier an electron has to overcome for coming out of the metal surface, the additional energy required to be imparted to an electron at absolute zero temperature to enable it to escape out will be equal to EB – EF. This additional energy i.e., EB – EF is called the work function of the metal and is designated as Ew or Φ that is
In other words, the work function (Ew or Φ) of a metal may be defined as the difference between the energy required to move an electron of a metal to an infinitely large distance and the maximum energy an electron can have at absolute zero temperature.
The work function of a metal depends upon the nature of the metal, and the surface conditions. Thus different metals have different work functions. Metals employed for electron emission should have low work function so that a small amount of energy is required for electron emission.
There arc several methods by which electron emission can be brought about. In all these methods the electrons arc supplied with energy in one form or the other in a quantity, from which they obtain kinetic energy sufficient to overcome the surface restraint.
Types of Electron Emission
- Thermionic Emission
- Secondary Emission
- Field Emission or Cold Cathode Emission
- Photoelectric Emission