Electron Emission:- As we know the valence electrons of the conductor atoms are loosely attached to the atomic nuclei. At room temperature, the thermal energy in the conductor is enough to break the bonds of the last valence electrons and leave them free to any one nucleus. These unbound electrons move at random within the conductor and are known as free electrons.
when an electric field is applied across the conductor, these unbound (free) electrons move through the conductor in an orderly manner, hence constituting the electric current. In this way, these free electrons move through the conductor or electric current flows through a wire.
As the operation of many electronic devices depends on the movement of electrons in an evacuated space. For this reason, the free electrons must be evicted from the surface of the metallic conductor by supplying enough energy from some external source. This is known as electron emission. The emitted electrons can be made to move in a vacuum under the influence of an electric field, thus constituting electric current in a vacuum.
The release of electrons from the surface of a substance is known as electron emission. Metals are used For electron emission because they have many free electrons. The random motion of free electrons is observed If a piece of metal is investigated at room temperature. though these electrons are free only to the extent that they may transfer from one atom to another within the metal they cannot depart from the metal surface to provide electron emission As shown in the below figure 2.1
It is because the free electrons that start at the surface of metal the nuclei positive charge pulling them back and none pulling forward. Hence at the surface of a metal, a free electron encounters forces that prevent it to leave the metal. In other words, the metallic surface offers a barrier to free electrons and is known as a surface barrier.
However, when sufficient external energy is applied to the free electron, its kinetic energy is increased and as a result, the electron will cross over the surface barrier to depart the metal. This additional energy needed by an electron to overcome the surface barrier of the metal is called the work function of the metal. The amount of additional energy needed to emit an electron from a metallic surface is known as the work function of that metal. Hence, if the total energy needed to release an electron from metal is 4 eV* and the energy already possessed by the electron is 0.5 eV, then additional energy needed (i.e., work function) is 4.0− 0.5 = 3.5 eV. The work function of pure metals changes roughly from 2 to 6 eV. It depends upon the nature of metal, the conditions of its surface, and its purity. The work function of metal used for electron emission should have low so that a small amount of energy is needed to cause emission of electrons.
The electron emission from the surface of a metal is possible only if enough additional energy (equal to the work function of the metal) is supplied from some external source. This external energy may come from a different sources such as the energy stored in the electric field, heat energy, light energy or kinetic energy of the electric charges bombarding the metal surface. Accordingly, there are following four principal methods are used to get electron emission from the surface of a metal.
In this method, the metal is heated to sufficient temperature (about 2500ºC) to enable the free electrons to depart the metal surface. These emitted free electrons depend upon the temperature. when the temperature is higher than the emission of electrons will be greater. This kind of emission is used in vacuum tubes.
In this method, a strong electric field (i.e. a high positive voltage) is supplied at the metal surface which attracts the free electrons out of metal because of the attraction of a positive field. This emission depends upon the field supplied, the stronger the electric field, the greater is the electron emission.
In this method, the energy of light falling upon the metal surface is transferred to the free electrons within the metal to enable them to depart the surface. This emission depends upon the intensity, the greater the intensity (i.e. brightness) of light ray falling on the metal surface, the greater is the photo-electric emission.
In this method, a high-velocity beam of electrons strikes the metal surface as a result of the free electrons of the metal to become out from the surface.
Definition: Thermionic emission is such a process in which electron emitted from a metal surface by supplying thermal energy to it is known as thermionic emission.
At ordinary temperatures, the energy possessed by free electrons in the metal is not enough to cause them to escape from the surface. When heat is applied to the metal, some of the heat energy is transformed into kinetic energy, causing accelerated motion of free electrons. When the temperature rises sufficiently, these electrons obtain additional energy equal to the work function of the metal. Consequently, they overcome the force of the surface barrier and leave the metal surface.
Metals with lower work function will need less additional energy so, for that, they will emit electrons at lower temperatures. The electron emission commonly used materials are tungsten, thoriated tungsten, and metallic oxides of barium and strontium. It may be noted here that high temperatures are necessary to cause thermionic emission. For instance, pure tungsten must be heated to about 2300ºC to acquire electron emission. However, oxide-coated emitters need only 750ºC for thermionic emission.
The amount of thermionic emission increases quickly as the emitter temperature is raised. The emission current density is specified by the Richardson-Dushman equation given below
The following points may be noted from above equation :
(i) The emission is noticeably affected by temperature changes. Doubling the temperature of an emitter may raise electron emission by more than 107 times. For example, emission from pure tungsten metal is about 10− 6 ampere per sq. cm. at 1300ºC but rises to a huge value of about 100 amperes when the temperature is raised to 2900ºC.
(ii) The small variation in the work function of the emitter can produce enormous effects on emission. likewise doubling the temperature, halving the work function has exactly the same effect.
The substance needed for electron emission is known as an emitter or cathode. The cathode is heated in an evacuated space to release electrons. If the cathode were heated to the needed temperature in the open air, due to the presence of oxygen in the air it would burn up.
A cathode should have the following properties:
(i) Low work function. The substance chosen as cathode should have low work function so that electron emission takes place by applying a small amount of heat energy i.e. at low temperatures.
(ii) High melting point. As electron emission takes place at very high temperatures (>1500ºC), so for that, the substance used as a cathode should have a high melting point. For a material such as copper, which has the benefit of a low work function, it is seen that it cannot be used as a cathode because it melts at 810ºC. As a result, it will vaporize before it begins to emit electrons.
(iii) High mechanical strength. The emitter should have high mechanical strength to bear up the bombardment of positive ions. In any vacuum tube, no matter how careful the evacuation, there is the formation of ions that collide with electrons when current flows. Under the presence of the electric field, the positive ions hit the cathode. If high voltages are used, the cathode is subjected to considerable bombardment and may be damaged.
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta
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