State the different modes of electron emission in metal. Explain any two modes of emission.

State the different modes of electron emission in metal. Explain any two modes of emission.
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1 Answer

Answer :

Different modes of electron emission in metal – 1. Thermionic emission 2. Photo electric emission. 3. Field emission. 4. Secondary emission.


Mode of Emission 

1. Thermionic emission. Concept: The electron emission from a metal surface, which occurs by supplying the thermal energy to the metal is called as thermionic emission. At room temperature, the free electrons of a metal do not possess sufficient energy to cause electron emission. As the temperature of a metal is increased, the electrons acquired energy more than the work function (Ø) due to which electrons gets accelerated and is emitted from surface of metal. 


Material : Tungsten, thorlated Tungsten, metallic oxides of barium and tungsten. 


Application: It is used in Vacuum tubes, cathode Ray tube (CRT), Camera tube, Picture tube in TV. 


2. Photo electric emission. The electron emission from the metal surface, when illuminated by light is called photo electric emission. When a beam of light is made to strike the surface of metal due to which the electron are emitted from its surface. The number of electrons emitted from the metal surface depends upon the intensity and frequency of incident light. Higher the intensity and frequency of light higher is the emission. These emitted electrons are called photo electrons. 


Material: Sodium, Potassium, Cesium, Rubidium 


Applications: It is used in Photo multiplier tubes, photo tube, Photo voltaic cell, Solar cells. 


3. Field emission. Concept: The electron emission from the surface of a metal which occurs due to application of strong electric field is high field emission. A strong electric field is applied at the surface of a metal, which pulls the electron out of the metal surface. The stronger the electric field, higher the electron emission. 


Material: Silicon, amorphous or diamond like carbon, Carbon nanowalls or wide band gap semiconductor. 


Applications: Electronic information display, Microwave generation, X-ray generation, Space vehicle neutralization. 


4. Secondary emission. Concept: When a solid surface is bombarded by electrons of adequately high energy, secondary electrons are emitted from the solid surface. The electrons being bombarded are called primary electrons the number of secondary electrons will depend on the energy of primary electrons. 


Materials: Magnesium oxide (MgO), Lead Oxide (PbO), Gallium posphide (GaP) 


Application: Electron multiplier tubes, Special amplifying tubes, Computer memory tubes. 

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