Operation:

Zero bias:

• At zero bias the diffusion of the holes and electrons across the junction causes space charge region of thickness inversely proportional to the impurity concentration.
• An ideal ‘i’ layer has no depletion region i.e. p layer has a fixed negative charge and n layer has a fixed positive charge.

Reverse bias:

• As reverse bias is applied the space charge regions in the p and n layers will become thicker.
• The reverse resistance will be very high and almost constant.

Forward bias:

• With forward bias carrier will be injected into the I layer and p and n space charge regions will become thinner.
• So the electrons and holes are injected into the i layer from p and n layers respectively. This increases the carrier concentration in the I layer above equilibrium. Thus resistivity decreases as increase in forward bias. Therefore low resistance is offered in the forward direction