Here, we will consider about the types of electron emission. But before that we have to know the definition of electron emissions.
Electron emission is state that the process of the liberation of free electrons from the metal surface of a substance is know as electron emission. Electron means negative charge particle and Emitting means producing.
Before reading about the different types of electron emission, we will have a little idea about what these free electrons are.
As you know, in 1897 JJ Thomson ( Joseph John Thomson ) discovered the electron. So now the question is what are electron or free electron ? We know that electrons revolve around the nucleus of an atom and in different orbits and there are some such substances of electrons in the last orbit which are know as valence electrons especially in metals ( Copper, Aluminum ). Valence electrons are so weakly attach to their nucleus that they can be easily remove or separate. Such electron are know as electron or free electron.
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Different types of electron emission
Depending on the way in which additional energy is given to the electron for electrons emission the electron emission is classify into four types.
- Thermionic Emission
- Photoelectric Emission
- Field Emission
- Secondary Emission
Thermionic emission
In this type of emission thermal energy or heat energy is use to increase the temperature of the material or metal. So due to this increase in temperature, then the energy level of the electrons also increase. Therefore these free electrons get additional amount of energy to be emitted from the metal surface.
Photoelectric emission
In this type of emission light energy is allow to fall on the metal surface. Because, this results in transfer of energy from the photons to the free electrons. This increased the energy level of the electrons and finally, they emitted from the surface.
Field emission
In the field emission a highly positive charged metal surface is place their reason is that closer to the metal. So, this result in a high intense electric field in between the two metal surface. therefore the free electrons get enough energy to be liberate from the surface.
Secondary emission
In this type of emission electrons from an electron source are allow to strike in to their metal surface. Because this collision of electrons results in transfer of energy. Then this transfer of energy increases the energy of electrons, which is helpful know as there emission.
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What is surface barrier in electronics ?
It is the barrier offered to the surface of the material to the emission of electrons from its surface is know as surface barrier. Because, this surface barrier is due to the attraction of electrons by the other atoms.
Define work function
The additional amount of energy that is essential by the free electrons to overcome the surface barrier and for the emission. Because to takes place is know as work function of that metal.
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As you know the free electrons can not leave the surface of metals. So the free electrons require external energy to leave the metal surface. Hence electron emission is achieve by various methods such as thermionic emission, photo-electric emission, secondary emission, field emission.