PN Junction

The PN junction is a fundamental concept in semiconductor physics, serving as the foundation for many electronic devices. A PN junction is formed when two types of semiconductors, p-type and n-type, are brought into contact. The p-type semiconductor contains excess positively charged carriers or holes. On the other hand, the n-type semiconductor contains excess negatively charged carriers or electrons. The importance of a PN junction lies in its ability to regulate the flow of electrical current, a feature exploited in various electronic applications.

Depletion Region

When p-type and n-type semiconductors are joined, a boundary region called the depletion region is created. The depletion region is a barrier for charge carriers due to immobile ions. These ions result from a process known as doping, where impurities are intentionally added to alter the conductivity characteristics of the material.

The depletion region consists of fixed positive charges on the n-side and fixed negative charges on the p-side. It creates an electric field across this region that opposes further movement of charge carriers. Once the depletion layer is formed, there is no net flow of charge across this junction. However, current can flow through the junction when a potential difference or voltage is applied.

Biasing of PN Junction

Biasing refers to the application of an external voltage across a PN junction. There are two main types of biasing: forward bias and reverse bias.

Forward Bias

Forward bias refers to the situation where the positive terminal of a battery connects to the p-type region, and the negative terminal connects to the n-type region of a PN junction. This configuration alters the depletion region’s characteristics.

Forward bias reduces the potential barrier across the PN junction. Consequently, this diminishes resistance and allows for a more effortless flow of charge carriers across both regions. With the potential barrier lowered, electrons from the n-type semiconductor move toward the p-type semiconductor, and holes from the p-type semiconductor move toward the n-type semiconductor. This movement of charge carriers constitutes an electric current in a diode, a simple semiconductor device based on a PN junction.

Reverse Bias

During reverse bias, the positive terminal of the battery is connected to the n-type semiconductor, and the negative terminal is connected to the p-type semiconductor. The external voltage enhances the potential barrier that naturally exists at the junction in an unbiased state. This increased potential barrier makes it more difficult for charge carriers to move across the junction.

As a result of this widening depletion region, it becomes more difficult for current to flow across the junction. The reverse bias creates an electric field that opposes or restricts current flow through the majority carriers. In practical terms, when a PN junction diode is reverse biased, only a small leakage current flows through due to minority carrier diffusion. This leakage current can be considered negligible compared to forward bias conditions. The reverse-biased PN junction exhibits high electrical resistance, and the diode is an insulator to the applied voltage.

I-V Characteristics of a PN Junction Diode

The I-V curve, or the current-voltage relationship, provides valuable insights into how a diode behaves under different biasing conditions. The graph typically shows the relationship between the forward or reverse bias voltage applied across the diode and the resulting current flowing through it.

At equilibrium, before any external bias is applied, a pn junction exhibits a depletion region near the junction that is depleted of majority carriers due to diffusion and creates a potential barrier. In this state, there is minimal current flow across the junction as the diffusion of carriers is balanced by the built-in potential.

Forward bias reduces the potential barrier. There is a slight increase in current initially. As the voltage increases, there is an exponential rise in current flow through the diode due to the growing number of injected carriers contributing to conduction. The forward-biased diode exhibits low resistance and allows a significant current flow.

Reverse bias increases the potential barrier. An increase in the potential barrier prevents the majority carriers from easily crossing the junction, resulting in minimal current flow. This saturation current is known as reverse leakage current. It occurs due to minority carriers (holes from the n-region and electrons from the p-region). However, as the reverse voltage is increased beyond a specific threshold, there is an abrupt increase in current, leading to a breakdown of the insulating properties of a semiconductor device. This phenomenon is often associated with the PN junction breakdown. The voltage at which breakdown occurs is known as the breakdown voltage. Our article on Zener diode discusses the different types of breakdown.