Analog Electronics / Diodes

Diodes: PN Junction, Biasing, V-I Curve, and Rectifier Action

A complete visual explanation of how a diode conducts, blocks current, enters breakdown, and works inside common analog circuits.

Try MCQs Download Notes Back to Analog Electronics

Diodes / Complete Concept

Step-by-Step Diode Explanation

Learn the diode from the inside out: PN junction formation, depletion barrier, forward bias, reverse bias, V-I graph, rectifier action, breakdown, and practical parameters.

Main Rule

Forward bias reduces the barrier and current flows. Reverse bias widens the barrier and current is blocked.

Exam Habit

First decide ON, OFF, or breakdown. Only then write KVL and calculate current.

AI Diagrams

Diode PN junction visualization

This circuit visualization is kept in AI Diagrams. Continue here with the topic explanation, working steps, and exam notes.

Open AI Diagrams

Core Idea

What Is a Diode?

A diode is a two-terminal semiconductor device that allows current to flow easily in one direction and strongly opposes current in the opposite direction.

The two terminals are the anode and the cathode. In normal forward conduction, conventional current enters through the anode, passes through the junction, and leaves from the cathode.

Anode is the positive-side terminal during normal forward bias.
Cathode is marked by the vertical line in the circuit symbol.
The diode behaves like a one-direction current controller, but only after the junction barrier is overcome.

Device Physics

PN Junction Formation

A diode is formed by joining p-type material, which has holes as majority carriers, with n-type material, which has electrons as majority carriers.

Immediately after joining, electrons diffuse from the n-side to the p-side and holes diffuse from the p-side to the n-side. Near the junction, this recombination leaves fixed ions behind, creating the depletion region.

The depletion region has almost no free carriers.
It acts like an internal barrier against further diffusion.
The internal electric field is what gives the diode its directional behavior.

Conduction Mode

Forward Bias

In forward bias, the anode is connected to the positive terminal and the cathode to the negative terminal. The applied voltage weakens the internal barrier.

As the depletion region shrinks, electrons and holes cross the junction more easily. Current then rises rapidly after the practical cut-in voltage.

Silicon diode forward drop is commonly approximated as 0.7 V.
Germanium diode forward drop is commonly approximated as 0.3 V.
Forward current must still be limited by a resistor or a circuit load.

Blocking Mode

Reverse Bias

In reverse bias, the anode is connected to the negative terminal and the cathode to the positive terminal. The external voltage strengthens the internal field.

The depletion region becomes wider, majority carriers are pulled away from the junction, and only a very small leakage current remains until breakdown.

Ideal reverse current is treated as zero.
Practical reverse current is a tiny leakage current.
If reverse voltage becomes too large, breakdown can occur.

Graph Reading

V-I Characteristic

The diode current-voltage curve is almost flat in reverse bias, then rises sharply in forward bias after the knee voltage.

The exponential equation explains the real device behavior, while exam circuits often use ideal or constant-voltage models to simplify analysis.

Forward region: current grows very fast after cut-in voltage.
Reverse region: current remains close to leakage level before breakdown.
Knee voltage is the practical point where conduction becomes significant.

Diode as a Rectifier

In a half-wave rectifier, the diode conducts during the positive half cycle and blocks during the negative half cycle. In a full-wave rectifier, the diode arrangement redirects both half cycles so the load current remains in one direction.

AI Diagrams

Rectifier waveform visualization

This circuit visualization is kept in AI Diagrams. Continue here with the topic explanation, working steps, and exam notes.

Open AI Diagrams

Breakdown and Parameters

Zener breakdown occurs at lower reverse voltages due to a strong electric field. Avalanche breakdown occurs at higher reverse voltages due to carrier multiplication. Practical diode selection depends on forward voltage, reverse breakdown voltage, current rating, and power rating.

Forward Voltage

Minimum practical voltage needed for conduction.

Breakdown Voltage

Reverse voltage where large reverse current begins.

Current Rating

Maximum safe current through the diode.

Power Rating

Maximum heat the diode can safely dissipate.

Important Diode Types

Rectifier diode

Converts AC to pulsating DC in power supplies.

Zener diode

Operates in reverse breakdown for voltage regulation.

LED

Emits light when forward biased.

Schottky diode

Switches fast and has a lower forward drop.

Photodiode

Converts light energy into electrical current.

Final Summary

A diode controls current direction because of the PN junction barrier. Forward bias shrinks the depletion region and allows conduction; reverse bias widens it and blocks current until breakdown. This single behavior creates rectifiers, regulators, clippers, clampers, LED indicators, photodetectors, and many protection circuits.