1. The electron tube depends for its action on a stream of electrons that act as current carriers. To produce this stream of electrons a special metal electrode (cathode) is present in every tube. But at ordinary room temperatures the free electrons in the cathode cannot leave its surface because of certain restraining forces that act as a barrier. These attractive surface forces tend to keep the electrons within the cathode substance, except for a small portion that happens to have sufficient kinetic energy (energy of motion) to break through the barrier. The majority of electrons move too slowly for this to happen.
2. To escape from the surface of the material the electrons must perform a certain amount of work to overcome the restraining surface forces. To do this work the electrons must have sufficient energy imparted to them from some external source of energy, since their own kinetic energy is inadequate. There are four principal methods of obtaining electron emission from the surface of the material: thermionic emission, photoelectric emission, field emission and secondary emission.
3. Thermionic emission. It is the most important and one most commonly used in electron tubes. In this method the metal is heated, resulting in increased thermal or kinetic energy of the unbound electrons. Thus, a greater number of electrons will attain sufficient speed and energy to escape from the surface of the emitter. The number of electrons released per unit area of an emitting surface is related to the absolute temperature of the cathode and a quantity of the work an electron must perform when escaping from the emitting surface.
4. The thermionic emission is obtained by heating the cathode electrically. This may be produced in two ways: 1) by using the electrons emitted from the heating spiral for the conduction of current (direct heating) or 2) by arranging the heating spiral in a nickel cylinder coated with barium oxide which emits the electrons (indirect heating). Normally, the method of indirect heating is used.
5. Photoelectric emission. In this process the energy of the light radiation falling upon the metal surface is transferred to the free electrons within the metal and speeds them up sufficiently to enable them to leave the surface.
6. Field or cold-cathode emission. The application of a strong electric field (i.e. a high positive voltage outside the cathode surface) will literally pull the electrons out of the material surface, because of the attraction of the positive field. The stronger the field, the greater the field emission from the cold emitter surface.
7. Secondary emission. When high-speed electrons suddenly strike a metallic surface they give up their kinetic energy to the electrons and atoms which they strike. Some of the bombarding electrons collide directly with free electrons on the metal surface and may knock them out from the surface. The electrons freed in this way are known as secondary emission electrons, since the primary electrons from some other source must be available to bombard the secondary electron-emitting surface.
EXERCISES:
1. Review questions:
1. What does the action of the electron tube depend on? 2. What is present in every tube to produce the stream of electrons? 3. At what temperatures free electrons cannot leave their surface of the cathode? 4. What forces tend to keep the electrons within the cathode substance? 5. What must the electrons do to escape? 6. What must the electrons have to overcome the restraining surface forces? 7. How many methods are there for obtaining electron emission? 8. What are they? 9. What imparts the external energy to the electrons in thermionic emission? 10. What energy is used for producing free electrons in photoelectric emission? 11. What is field emission?
12. How is secondary emission obtained? 13. What emission is the most commonly used in electronics?