V-I Characteristics of Diodes

Definition

The V-I characteristics of a diode are the graphical representation of the current through a diode as a function of the voltage across it, showing how the diode conducts in forward bias, blocks in reverse bias, and behaves during breakdown.

Main Content

1. Diode Structure and Biasing

A diode is formed by joining a p-type semiconductor with an n-type semiconductor, creating a p-n junction.
The junction region contains a depletion layer with immobile ions, which creates a barrier potential that controls current flow.
In forward bias, the p-side is connected to the positive terminal and the n-side to the negative terminal, reducing the barrier and allowing current to flow.
In reverse bias, the p-side is connected to the negative terminal and the n-side to the positive terminal, increasing the barrier and preventing current flow.
The V-I characteristics depend directly on this biasing condition, which determines whether the diode conducts or blocks current.

2. Forward Bias Characteristics

When a forward voltage is applied, the external supply opposes the built-in barrier potential, making it easier for majority carriers to cross the junction.
Initially, the current is very small because the applied voltage must first overcome the cut-in or threshold voltage.
For a silicon diode, the cut-in voltage is approximately 0.7 V, and for a germanium diode it is approximately 0.3 V.
After this threshold, the current increases rapidly and becomes very large for a small increase in voltage, producing an exponential rise in the graph.
The forward current is mainly due to majority carriers, and the diode exhibits very low dynamic resistance in this region.
Example: In a silicon rectifier circuit, once the input exceeds 0.7 V, the diode starts conducting heavily and allows current through the load.

3. Reverse Bias Characteristics and Breakdown

In reverse bias, the depletion region widens and the barrier potential increases, so only a very small current flows.
This small current is called reverse saturation current or leakage current, and it is due to minority carriers.
The reverse current remains almost constant over a wide range of reverse voltage, showing that the diode effectively blocks current.
If the reverse voltage is increased beyond a certain limit, the diode enters breakdown region.
Breakdown may occur by Zener breakdown in heavily doped diodes at low voltage or avalanche breakdown in lightly doped diodes at higher voltage.
In breakdown, the reverse current increases sharply, and if not limited, the diode may get damaged.
Example: Zener diodes are designed to operate safely in the breakdown region and are used for voltage regulation.

Working / Process

1. Apply Forward or Reverse Bias

Connect the diode to an external voltage source either in forward bias or reverse bias to study its behavior.

2. Measure Voltage and Current

Increase the applied voltage step by step and observe the corresponding current through the diode in both directions.

3. Plot the V-I Curve

On a graph, take voltage on the x-axis and current on the y-axis to obtain the diode characteristic curve, showing the forward conduction region, reverse blocking region, and breakdown region.

Advantages / Applications

The V-I characteristics help in understanding and designing rectifiers, where diodes convert AC to DC.
They are essential in switching circuits, because a diode can act as an ON-OFF electronic switch depending on bias.
They are useful in voltage regulation and protection circuits, especially with Zener diodes and reverse breakdown control.
They help engineers select the correct diode for a circuit by considering cut-in voltage, reverse leakage current, and breakdown voltage.

Summary

A diode conducts easily in forward bias and blocks current in reverse bias.
Its V-I curve shows cut-in voltage, forward conduction, reverse saturation current, and breakdown behavior.
The characteristics are essential for analyzing and designing practical electronic circuits.
V-I characteristics of a diode provide the basic understanding of how semiconductor devices control current flow.