Question

(a) Describe briefly, with the help of a diagram, the role of the two important processes involved in the development of a P-N junction.
(b) Name the electronic device which is used as a voltage regulator. Draw the needed circuit diagram and explain its working.

Answer 1

The two important processes involved in development of P-N junction. They are diffusion and drift. In these processes, the diffusion current and the drift current are produced respectively. They are in opposite directions also. The voltage regulator does the job of keeping a constant voltage in a circuit.

Complete step-by-step answer :

(a) When the P side and the N side are coming in contact then majority charge carriers will diffuse from a higher concentration to lower concentration. This will trigger the electrons to flow from N to P junction and there it gets combined with holes near the junction. In the same way, holes will be flowing from P side to the N side and combine with the electrons in the junction. When this occurs, the region around the junction gets depleted of carriers and gets charged as we can see in the figure. This diffusion of charge carriers is known as diffusion current. The charge in the depletion region develops an electric field. This electric field will be the reason for the flow of minority charge carriers across the depletion region. This is known as the drift current. Both this diffusion and drift currents are in the opposite direction. When there is equilibrium, they are equivalent to each other and thus the P-N junction is formed.

(b) Zener diode is the device used as a voltage regulator. When the unregulated voltage is connected to a zener diode by adding a series resistor to it, such that the zener diode is reverse biased. When the input voltage is enhanced, the current through the zener diode and resistance also get increased. Therefore the voltage drop over resistance also gets enhanced without the variation in voltage drop over zener diode. This is due to zener voltage remaining the same irrespective of the current in the breakdown region. In the same way when the input voltage decreases, voltage and current in the resistance will be reduced without changing zener diode voltage.

Note : Zener diodes are helpful in voltage regulation. They are useful as surge suppressors, in switching processes and clipper circuits. They are also used as reference elements. They can be used for the process of rectification also.