A diode is a fundamental two-terminal semiconductor device that allows electric current to flow primarily in one direction while blocking current in the opposite direction. Because of this property, it is widely used in rectification, switching, signal detection, and voltage regulation circuits.
Table of Contents
Structure of a Diode
A diode is formed by joining P-type semiconductor and N-type semiconductor materials to create a PN junction.
• P-region contains holes as majority charge carriers
• N-region contains electrons as majority charge carriers
When these regions are joined, a depletion region is formed at the junction due to recombination of electrons and holes. This region acts as a potential barrier that prevents free movement of charge carriers.
Symbol and Terminals
A diode has two terminals:
• Anode (P-side)
• Cathode (N-side)
Current flows from anode to cathode when the diode is forward biased.
Biasing of a Diode
Forward Bias
Forward bias occurs when:
• Positive terminal of the battery is connected to the P-region
• Negative terminal is connected to the N-region
Effects of forward bias:
• The depletion region narrows
• Barrier potential reduces
• Current flows easily through the diode
Typical forward voltage:
• Silicon diode: ≈ 0.7 V
• Germanium diode: ≈ 0.3 V
Reverse Bias
Reverse bias occurs when:
• Positive terminal is connected to the N-region
• Negative terminal is connected to the P-region
Effects of reverse bias:
• The depletion region widens
• Barrier potential increases
• Only a very small leakage current flows
If the reverse voltage becomes very high, breakdown may occur.
Diode Current Equation
The current through a diode is given by the Shockley diode equation:
\[
I = I_s \left(e^{\frac{V}{\eta V_T}} – 1\right)
\]
where
\(I\) = diode current
\(I_s\) = reverse saturation current
\(V\) = applied voltage
\(V_T\) = thermal voltage
\(\eta\) = emission coefficient (typically 1 to 2)
V–I Characteristics of a Diode
The voltage-current characteristics of a diode show how current varies with
applied voltage.
Forward Region
Current increases rapidly after the cut-in voltage.
Reverse Region
Only a small reverse saturation current flows.
Breakdown Region
At a certain reverse voltage, current increases sharply due to
Zener or avalanche breakdown.
Types of Diodes
Common types of diodes include:
• Rectifier diode
• Zener diode
• Light-emitting diode (LED)
• Photodiode
• Schottky diode
• Varactor diode
• Tunnel diode
Each type is designed for specific electronic applications.
Applications of Diodes
Diodes are used in many electronic circuits, including:
• Rectifiers in power supplies
• Signal detection in communication systems
• Voltage regulation using Zener diodes
• Switching circuits
• Light generation in LEDs
• RF circuits and mixers
Conclusion
The diode is one of the most basic and widely used semiconductor devices in
electronics. By allowing current to flow in only one direction, it plays a
crucial role in rectification, signal processing, and voltage regulation.
Understanding the structure, operation, and characteristics of diodes is
essential for analyzing and designing electronic circuits in modern
communication and control systems.