Question

Draw the circuit arrangement for studying the V-I characteristics of a p-n junction diode (i) in forward bias and (ii) in reverse bias. Draw the typical V-I characteristics of a silicon diode.
Describe briefly the following terms :
(i) " minority carrier injection " in forward bias
(ii) " Breakdown voltage " in reverse bias.

Answer 1

(Refer Fig. 1) Two processes occurring the formation of a p-n junction are diffusion and drift. Due to the concentration gradient across p and n sides of the junction , holes diffuse from p-side to n-side ( p $$ \rightarrow $$n)and electrons diffuse from n-side to p-side (n$$\rightarrow$$p) . This movement of charge carriers leaves behind ionised acceptors (negative charge $$ \phi $$-immobile ) on the p-side and donors (positive charge immobile) on the n-side of the junction. This space charge region on either side of the junction together is known as depletion region.

(b) (Refer Fig. 2) The circuit for studying the V-I characteristics of a diode are shown in Fig.(a) and Fig.(b) . For different values of voltages the value of current is noted . A graph between C and I is obtained as in Fig. (c).

From the V-I characteristics of a junction diode it is clear that it allows current to pass only when it is biased. So if an alternating voltage is applied across a diode the current flows only in part of the cycle when the diode is forward biased. This property is used to rectify alternating voltages.

(i) Minority Carrier Injection : Due to the applied voltage , electrons from n-side cross the depletion region and reach p-side (where they are minority carriers) . Similarly , holes from p-side cross this junction and reach the n-side *where they are minority carriers). This process under forward bias is known as minority carrier injection.

(ii) Breakdown voltage : It is a critical reverse bias voltage at which current is independent of applied voltage.