Tutorial Lecture on The Theory of Electron Emission

Basic Concept of Electron Emission

The phenomenon of emitting electrons from the material surface into surrounding by using a specific external energy is called the Electron Emission. As we that every metal is made of those atoms which are connected in crystal shape very tightly to each other. All electrons are bounded by their atoms effect. Electronic devices use free electrons to flow the current through itself. Valence electrons are those electrons which are revolving around in their last shell. These electrons are bounded to the metal. It requires sufficient external energy to escape from the material surface. Now the maximum external energy requires for the surface electrons to emit from a material surface at 0c° is called work function.

Learn how to investigate Emission of Electrons from the body surface 

                  Electrons emission can be achieved through different external methods according to the situation and different devices. The most common and simple technique used for electron emission is thermionic emission / primary emission. In this method, thermal energy is applied to the surface electrons of material. By absorbing thermal energy the kinetic energy of the electrons gets increase and restraining force of the electrons is now getting low and low due to the change in electrons kinetic energy. This energy emission method is commonly use applied to metals when it is put into the vacuum or inert gases. Forgetting high-efficiency thermionic emitter must be of low work function so that it can be operated at low temperature. The second method of electron emission is achieved through a mechanical collision of electrons. When a moving particle (primary electron) with some velocity strikes with the surface electrons of a material, the moving electron transfers its energy to the stationary electron with the physical collision. This in result increases the kinetic energy of surface electron which tends to move from the surface called a secondary electron. Secondary emission can be affected by some factors like primary electrons energy, Emitting material, Striking electrons mass, Type of primary electrons, Target material surface and Striking angle. See below figures respectively.
Emission of an Electron from the Metalic surface 

Similarly, the electron emission can be done through light effect. This method is widely used in phototubes. When light is incidence on a light-sensitive material (potassium, radium etc), it’s surface electrons absorb light energy which in result increase the kinetic energy of an electron and starts to emit above the surface. This particular technique of electron emission is known as Photoelectric emission. Some key factors involve in photoelectric emission which can affect the process i-e photoelectric emission starts & stops directly with the incidence of light on the surface, radiation frequency, photoelectron emission requires different light frequency for different materials and emitted electron’s kinetic energy is independent of light flux. The light frequency must not be less than the threshold frequency. Also there is another technique used for electron emission called field emission/cold cathode emission, as we know that electrons are negatively charged particles, So if we bring high positive field near to the surface of a material, the surface electrons will absorb that energy of attraction and the electrons tend to move from material surface to the surrounding. As high we provide positive electric field outside electron emission will be that much high. If the external field intensity goes up to 

106 v/cm then the electron emission rate will increase enormously. Another point to remember is cold cathode emission doesn’t depend on temperature but the applied positive potential must be high than a threshold voltage. All electron emission processes are used in a different device like cathode ray tubes, X-rays, medical types of equipment etc. Following the processes see the below diagrams.

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