Question

What happens when a p-n junction is (a) forward biased (b) reverse biased?

Answer 1

When a p-n junction is (a) Forward Biased:
The effective barrier potential reduces i.e. ($V_o-V$) where $V_o$ = barrier potential initially, V = forward voltage applied. Therefore, the thickness of depletion layer also decreases. The junction resistance becomes very low. The holes from p region move to n-side and electrons from n-side move towards p-side. The movement of holes and electrons constitute hole current ($I_h$) and diffusion electron ($I_e$). Total current $I=I_e+I_h$.
(b) Reverse Bias: When a voltage V (i.e. reverse bias voltage) is applied to a circuit. The effective barrier potential increases, it becomes ($V_o+V$). Also the thickness of depletion layer increases. The junction resistance increases. The majority of carriers in p and n-regions respectively are attracted by negative and positive terminals of the battery. Therefore, they cannot move across the junction. There is a small saturation current due to the sweep of minority carrier in p and n-regions.