Electron emission

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:

DIODES

1. The simplest combination of elements constituting an electron tube is the diode. It consists of a cathode, which serves for emitting the electrons, and a plate or anode sur­rounding the cathode, which acts as a collector of electrons. Both electrodes are enclosed in a highly evacuated envelope of glass or metal. If the cathode is indirectly heated, there must be a heating spiral or a heater. The size of diode tubes varies from, tiny metal tubes to large-sized rectifiers. The plate is generally a hollow metallic cylinder made of nickel, molybdenum graphite, tantalum or iron.

2. A basic law of electricity states that like charges repel each other and unlike charges attract each other. Electrons emitted from the cathode of an electron tube are negative electric charges. These charges may be either attracted to or repelled from the plate of a diode tube, depending on wheth­er the plate is positively or negatively charged.

3. Actually, by applying a potential difference (voltage) from a battery or other source between the plate and cathode of a diode, an electric field is established within the tube. The lines of force of this field always extend from the negatively charged element to the positively charged element. Electrons, being negative electric charges, follow the direction of the lines of force in an electric field.

4. By establishing an electric field of the correct polari­ty between cathode and plate and "shaping" the lines of force of this field in certain paths,¹ the motion of the electrons can be controlled as desired. A battery is connected between plate and cathode of a diode, so as to make the plate positive with respect to the cathode, the lines of force of the electric field extending in a direction from the cathode to the plate.

5. Again, applying a heater voltage results in emission of electrons from the cathode. The electrons follow the lines of force to the positive plate and strike it at high speed. Since moving charges comprise an electric current, the stream of electrons to the plate is an electric current, called the plate current.

6. Upon reaching the plate the electron current continues to flow through the external circuit made up of the connect­ing wires and the battery. The arriving electrons are absorbed into the positive terminal of the battery and an equal number of electrons flow out from the negative battery ter­minal and return to the cathode, thus replenishing the sup­ply of electrons lost by emission.

7. As long as the cathode of the tube is maintained at emitting temperatures and the plate remains positive, plate current will continue to flow from the cathode to the plate within the tube and from the plate back to the cathode through the external circuit.

8. Now a battery connection has been reversed so as to make the plate negative with respect to the cathode. When voltage is applied to the heater the cathode will emit a flow of electrons. However, these electrons are strongly repelled from the negatively charged plate and tend to fill the inter-electrode space between cathode and plate. Since no electrons actually reach the plate, the tube acts like an open circuit.

9. The total number of electrons emitted by the cathode of a diode is always the same at a given operating tempera­ture. The plate voltage (voltage between plate and cathode) has no effect, therefore, on the amount of electrons emitted from the cathode. Whether or not these electrons actually reach the plate, however, is determined by the plate-to-cath­ode voltage,² as well as by a phenomenon known as space charge.

10. The term space charge is applied to the cloud of elec­trons that is formed in the interelectrode space between cath­ode and plate. Since it is made up of electrons, this cloud constitutes a negative charge in the interelectrode space that has a repelling effect on the electrons being emitted from the cathode. The effect of this negative space charge alone, there­fore, is to force a considerable portion of the emitted elec­trons back into the cathode and prevent others from reaching the plate.

11. The space charge, however, does not act alone. It is counteracted by the electric field from the positive plate, which reaches through the space charge to attract electrons and thus partially overcomes its effects. At low positive plate voltages only electrons nearest to the plate are attracted to it and constitute a small plate current. The space charge then has a strong effect on limiting the number of electrons reaching the plate.

12. As the plate voltage is increased, a greater number of electrons are attracted to the plate through the negative space charge and correspondingly fewer are repelled back to the cathode. If the plate voltage is made sufficiently high, a point is reached eventually, where all the electrons emitted from the cathode are attracted to the plate and the effect of the space charge is completely overcome. Further increases in the plate voltage cannot increase the plate current through the tube, and the emission from the cathode limits the maxi­mum current flow.