P-N Junctions: Behavior under Forward and Reverse Bias

P-N Junctions

A pn junction is formed by combining two types of semiconductor material: (n)-type semiconductor doped with donor impurities and (p)-type semiconductor doped with acceptor impurities. When these regions meet, they form a boundary called a pn junction. This junction exhibits certain properties due to the diffusion of charge carriers across it, mainly the depletion region, electric field, and built-in potential.

In pure semiconductors, there are equal numbers of electrons and holes. However, when impurity atoms are introduced into the crystal lattice, they attract or repel electrons depending on their nature, which changes the carrier concentration. In the formation of a pn junction, the (p)-region has more holes than electrons, and the (n)-region has more electrons than holes. Thus, there is an excess of negative charges in the (n)-region and deficit of positive charges in the (p)-region.

The distribution of the excess negative and deficit positive charges gives rise to an electric field, known as the built-in field, directed from the (n)-region towards the (p)-region. This electric field helps maintain equilibrium between the (n) and (p) regions, preventing the flow of current through the junction under normal conditions.

Forward Bias

When a voltage is applied such that the (+) terminal is connected to the (p)-region, and the (-) terminal is connected to the (n)-region, the built-in field is reduced. This is known as forward bias.

When a forward bias is applied, the depletion region is reduced in thickness. As the voltage is increased, the depletion region eventually disappears, and a large current flows through the junction, passing through the (n)-type material. The current flow in forward bias can be described as the diffusion of majority charge carriers and the drift of minority charge carriers across the junction.

Reverse Bias

Applying a voltage such that the (+) terminal is connected to the (n)-region, and the (-) terminal is connected to the (p)-region, increases the built-in field. This condition is called reverse bias.

In reverse bias, the depletion region is increased, and the current flow across the junction is reduced or even blocked. The minority charge carriers are repelled by the increased electric field, causing a reverse flow of majority carriers. This flow of majority carriers in reverse bias is known as minority carrier injection.

In summary, the behavior of a (pn) junction under forward and reverse bias conditions plays a crucial role in various electronic devices like diodes and transistors. Understanding these principles allows us to design and control the operation of these semiconductor components effectively.