Semi – Conductor Diode – When the p and n-regions of a p-n junction are connected to metallic wires and enclosed in a glass or a metallic case in such a way that only the wires connected to n a and p-regions project outside the case, then such an encapsulated p-n junction is known as a diode. Its p part is known as node and n part as cathode as shown fig. 19. 10(a). Fig. 19.10(b) shows the symbolic representation of a diode.
Under ordinary conditions, no current passes through a diode due to its internal potential barrier. If an external potential source, for example, a battery is connected across the junction so that current begins to flow through it, the diode is said to be forward biased. For this two conditions are to be satisfied.
- The p-junction of the anode of the diode is connected with the positive terminal of the battery and its cathode with the negative terminal of the battery as shown in Fig. 19. 11.
- The potential of the battery should be greater than the potential barrier of the diode so that the battery could provide the requisite amount of energy to charge carriers present in the p-n junction to overcome the potentials barrier.
A specific amount of current is allowed to flow through a diode. If the current passing through the diode exceeds this specific value, the diode gets heated and is likely to be damages. Therefore, in the biasing circuit of fig. 19. 11, a resistance R has been shown which limits the current passing through the diode within its specific value.
When a diode is forward biased, the width of its depletion region decreases. It means that the quantity of positive and negative ions present in the depletion region becomes less due to which the potential barrier also gets low and the current begins to flow easily through the junction. In this situation.
The resistance region of an unbiased diode has been shown by dotted lines, ordinary lines. Similarly, in Fig. 19.12 (b) , the potential barrier of an unbiased diode has been shown by dotted lines, whereas that of a forward bias diode has been shown by ordinary lines.
Reverse biased diode
When the p part of the diode, i.e., its anode is connected with the negative terminal and its cathode with the positive terminal of a battery, the diode is said to be reverse biased as shown in Fig. 19.13 (a). in this state, the current passing through the diode is nearly zero and we can say that in general, no current flows through a reverse biased diode.
It is because of the fact that in reverse biased state, the width of the depletion region increases due to which the potential barrier becomes so high that the charge carriers are unable to over come it. Therefore, no current passes through the junction. In this state, the resistance of the junction or the diode becomes equal to several mega ohms. Like Fig. 19. 12, the depletion region and potential barrier of a reverse biased diode is shown in fig. 19. 13 (b,c).