Question

(a) State briefly the processes involved in the formation of p-n junction explaining clearly how the depletion region is formed.
(b) Using the necessary circuit diagram, show how the V-I characteristics of a p-n junction are obtained in
(i) Forward biasing
(ii) Reverse biasing
How are these characteristics made use of in rectification?

Answer 1

(a)

When an electron diffuses from n to p due to difference in charge carriers of the two regions of p-n junction, it leaves behind a positive charge (ionised donor) on n side which is immobile as it is bonded by surrounding atoms. Thus a layer of positive and negative charges develop on the n-side and p-side of the junction respectively. Hence a space charge region is formed on both side of the junction which is called as depletion layer or depletion region.

(b)

(i) In Forward bias, p-side of the diode is connected to the positive terminal of the battery and n-side of the diode is connected to the negative terminal of the battery. Applied voltage is slowly increased and the current in the circuit is measured and plotted. It is observed that beyond the knee voltage, current rises rapidly.

(ii) In reverse bias, n-side of the diode is connected to the positive terminal of the battery and p-side to the negative terminal. Applied voltage is slowly increased, it is observed that almost non current flows in the circuit until a substantial reverse breakdown voltage. Beyond this point, current rises suddenly and diode works in reverse breakdown region.

A diode hence passes only positive values of voltage and blocks negative values of voltage. When, applied across an ac source it passes the positive cycles and blocks the negative cycles. Hence, it is used for rectification purposes.