A PN junction is a fundamental building block in semiconductor devices, particularly diodes. It is formed by joining a P-type semiconductor (with excess holes) and an N-type semiconductor (with excess electrons). When these two regions come together, the free electrons from the N-side diffuse to the P-side, resulting in a region depleted of majority carriers near the junction, called the depletion region.

The behavior of diodes is based on this PN junction. In a forward biased configuration, where the P-side is connected to the positive terminal of a voltage source and the N-side to the negative terminal, the diode allows current to flow. This is because the positive potential on the P-side repels the holes, while the negative potential on the N-side attracts the electrons, reducing the width of the depletion region and enabling current flow.

On the other hand, in a reverse biased configuration, where the P-side is connected to the negative terminal and the N-side to the positive terminal, the diode prohibits current flow. The positive potential on the P-side attracts the electrons, while the negative potential on the N-side repels the holes, widening the depletion region and blocking the flow of majority carriers.

Diodes find applications in various electronic circuits, such as rectifiers to convert AC to DC, as voltage regulators, and in signal processing circuits. They provide the ability to control the direction of current flow by capitalizing on the properties of the PN junction.