Question

(a) Describe briefly, with the help of a diagram, the role of the two important processes involved in the formation of a $p-n$ junction.
(b) Name the device which is used as a voltage regulator. Draw the necessary circuit diagram and explain its working.

Answer 1

The two important process involved in formation of p - n junction:

(i) Diffusion

(ii) Drift

When p and n sides comes in contact then majority carries diffuse from high concentration to low concentration. Electrons flow from n to p junction and combines with holes near the junction. Similarly holes flow from p-side to n side and combine with electrons near the junction. As this happens, region near the junction gets depleted of carriers and gets charged as shown in figure. This diffusion of carriers leads to diffusion current.

The charge in depletion region creates electric field. This electric field leads to flow of minority carriers across depletion region. This is called the drift current.

The diffusion and drift currents are opposite in direction.

In equilibrium, they are equal to each other and p-n junction is formed.

(B) Zener diode is used as a voltage regulator.

The unregulated voltage is connected to zener diode through a series resistance such that the zener diode is reversed biased. If the input voltage increases, the current through the zener diode and resistance increases.

Thus the voltage drop over resistance increases without the change in voltage drop over zener diode. This is because in the breakdown region, zener voltage remain same irrespective of the current.

Similarly, if the input voltage decreases, voltage and current over resistance decreases without change in voltage across the zener diode.