Explain forward bias and reverse bias of the junction diode.

Explanation

(i) Forward biasing of P-N junction diode. When the positive terminal of battery is connected to p-region and the negative terminal is connected to n-region of p-n junction diode then the junction diode is said to be forward biased.

Due to force of repulsion from the battery, both holes and the electrons from the P-region and N-region respectively move towards the junction and combine after crossing the junction. For each hole-electron combination, a covalent bond in p-section near the positive terminal of the battery is broken and the electron generated enters the positive terminal of the battery and hole remains there in p-section. The hole moves across the junction due to applied field. Simultaneously an electron from the negative pole of the battery enters n-region of junction-diode compensating the loss of electron during combination. This recombination of hole-electron appears as forward current. A small increase in forward voltage results large increase in forward current. Hence diode offers small resistance when forward biased.
(ii) Reverse biasing of p-n junction. If the positive terminal of battery is connected to n-region and negative terminal of battery is connected to p-region of junction -diode, then the diode is said to be reverse biased. In this case holes are attacted by the negative terminal of battery and electrons are attracted by the positive terminal of battery and as a result both move away from the junction and hence no current flows due to majority charge carriers. This results in the increase in width of depletion layer. But a very small current flows due to minority charge carriers, which is known as reverse current. Since minority charge carriers are thermally generated, therefore reverse current is temperature dependent.