External strong electric field nearer to the surface of the emitter can affect the emission of electrons. A free electron at the extreme surface of the metal can’t escape the surface due to back pull from the positive nuclei in the emitter body. This back pull on a free electron causes a potential barrier on the electron. An electron has to overcome this potential barrier to get emitted from the emitter surface. In more details we can say, a free electron well inside the emitter body feels alteration force from all sides due to the presence of positive nuclei surrounding it. But at the extreme edge of the surface of emitter body, the free electron feels only the alternative force from the nuclei behind it. Since there is no nucleus ahead of the free electron to attract it in the outward direction.
Due to this electrostatic alteration force on a free electron, it has to aquire sufficient kinetic energy to overcome this free for escaping from the surface of the emitter. In other words, there will be a potential barrier for that electron created on the surface of the emitter. This potential barrier is also known as the surface barrier. A free electron has to overcome this surface barrier for getting emitted.
But when a sufficiently high positive charge is placed in front of the emitter surface, due to the strong electrostatic force of the created electric field, the free electron can get sufficient energy to overcome the surface barrier and can get emitted from the surface of the emitter body. As this type of electron emission is caused by the electric field present in the space, it is called field emission.
Definition of Field Emission
Description of Field Emission
When we place a metal surface in front of a conductor of high positive potential, with respect to the potential of metal surface, the strong electric field exerts a force on the free electrons. As the force applied to the free electrons is strong enough, the free electrons overcome the restraining force (i.e. surface barrier) and come out from the metal surface.
In emission process, a very intense electric field needs to be created in the space. A million volts per centimetre would be the voltage gradient of the field. In field emission process, the emitter may not need any extra heating for the field emission and that is why the process is also referred to as cold cathode emission.