V-I Characteristics of PN Junction

Definition

The V-I characteristics of a PN junction refer to the graph and behavior of current flowing through a PN junction diode as a function of the applied voltage across it, showing how the diode conducts current in forward bias and blocks current in reverse bias under different voltage conditions.

Main Content

1. PN Junction Diode and Biasing

A PN junction diode is made by joining a p-type semiconductor, rich in holes, with an n-type semiconductor, rich in electrons. At the junction, electrons and holes diffuse across the boundary and recombine, creating a region near the junction called the depletion layer or depletion region.
Because of this charge separation, a built-in potential barrier is formed that opposes further diffusion of charge carriers. The behavior of the diode depends on whether the external voltage is applied in forward bias or reverse bias. In forward bias, the p-side is connected to the positive terminal and the n-side to the negative terminal, reducing the barrier. In reverse bias, the p-side is connected to the negative terminal and the n-side to the positive terminal, increasing the barrier.

2. Forward Bias Characteristics

In forward bias, when a small voltage is applied, the depletion layer becomes narrower and the potential barrier reduces. However, the current remains very small initially because the applied voltage is not yet sufficient to overcome the barrier effectively.
After the cut-in voltage or threshold voltage is reached, the current increases very rapidly with a small further increase in voltage. For a silicon diode, this is approximately 0.7 V, and for a germanium diode, it is approximately 0.3 V. Beyond this point, the diode conducts heavily, and the V-I graph shows a steep rise in current. This region is used in practical circuits because the diode allows current to pass efficiently once the barrier is overcome.

3. Reverse Bias Characteristics

In reverse bias, the depletion region widens and the potential barrier increases, preventing the majority charge carriers from crossing the junction. As a result, only a very small reverse saturation current flows due to minority carriers.
This reverse current remains nearly constant even when the reverse voltage is increased, until a critical reverse voltage called breakdown voltage is reached. At breakdown, current increases sharply. Breakdown may occur by the Zener effect in heavily doped junctions or by avalanche breakdown in lightly doped junctions. This property is important in voltage regulation and protection circuits.

Working / Process

When the PN junction is unbiased, charge diffusion near the junction creates a depletion region and a built-in electric field that prevents continuous carrier flow.
When forward bias is applied, the external voltage opposes the built-in field, reducing the barrier height and allowing majority carriers to cross the junction, producing a large forward current after the threshold voltage.
When reverse bias is applied, the external voltage strengthens the barrier, blocking majority carriers and allowing only a small minority carrier current until breakdown occurs at high reverse voltage.

Advantages / Applications

The V-I characteristics help in understanding how a diode controls current flow, making the PN junction useful as a rectifier in AC to DC conversion.
The sharp change in current after threshold voltage makes the diode useful in switching circuits, clipping circuits, clamping circuits, and signal detection.
The reverse breakdown region is used in Zener diodes for voltage regulation and overvoltage protection in electronic circuits.

Summary

The V-I characteristics of a PN junction show the current-voltage behavior of a diode in forward and reverse bias.
In forward bias, current is very small initially and then rises rapidly after the cut-in voltage.
In reverse bias, only a small saturation current flows until breakdown occurs.
Important terms to remember: PN junction, depletion region, potential barrier, forward bias, reverse bias, cut-in voltage, reverse saturation current, breakdown voltage.