PN junction diode forward and reverse bias

A PN junction diode is a type of semiconductor device that allows current to flow in only one direction, from the p-type material to the n-type material. The flow of current in a PN junction diode depends on the voltage applied across it, which determines the bias of the diode.

In this article we will try to understand forward and reverse bias of pn junction diode. Because the understanding of the behavior of PN junction diodes in forward and reverse bias is important in many electronic applications of pn junction, such as rectification, voltage regulation, and switching. And also help us to understand the characteristics of pn junction diode.

2 Types of bias in pn junction diode

• Forward biased pn junction diode.
• Reversed biased pn junction diode.

Forward biased pn junction diode

When a battery of an e.m.f.  greater than the barrier potential (Vb) is connected to a PN junction diode in such a way that the positive terminal of the battery is connected to the p-region and the negative terminal of the battery is connected to the n-region of the junction diode, then the PN junction diode is said to be forward-biased.

Thus, the flow of electrons to the left side and hole to the right side of the junction begin. The movement of holes and electrons constitute hole current (Ih) and electron current (Ie) respectively.

In the region of the p-n junction, electrons and holes recombine. For every electron-hole combination near the junction, a covalent bond of a p-type semiconductor connected to the positive terminal of the battery breaks. 

The electron liberated enters the positive terminal of the battery and the hole moves to the right side of the junction. Similarly, more electrons from the negative terminal of the battery enter the n-region to compensate for the electron lost by the combination with the hole at the junction.

These electrons diffuse through the junction and enter the p-region. Here, they again combine with the holes. Thus a continuous current flows through the junction diode. 

Reversed biased pn junction diode

A PN junction is said to be reversed biased when the positive terminal of the battery is connected to the n-region and the negative terminal of the battery is connected to the p-region of the PN junction diode.

Hence the junction resistance increases. The holes and electrons (majority carriers) in the p-region and n-region respectively are attracted by the negative and positive terminal of the battery.

Thus, both holes and electrons are displaced away from the junction. As a result of this, holes in the p – region and electrons in the n – region cannot cross through the junction. Therefore, the flow of current in the diode is almost stopped.

There is a small current due to the minority carrier in the p-region and n-region, which exists under reverse-bias conditions. The current is called saturation current and denoted by Is. This current is not affected by the applied voltage but increases with the increase in temperature.

FAQ’s on PN junction diode forward and reverse bias