In this lecture, we study the PN Junction, understand how a diode works under forward and reverse biasing, and analyze the I–V characteristics of a PN junction diode.

🔷 What is a PN Junction (Diode)?

A PN junction is formed when a P-type semiconductor is joined with an N-type semiconductor.

When joined:

Electrons from N-region diffuse into P-region
Holes from P-region diffuse into N-region
This creates a depletion region
An internal electric field is formed
A potential barrier (junction potential) develops

This device is called a diode, which allows current to flow mainly in one direction.

⚡ Forward Biasing

In forward bias:

P-side is connected to positive terminal
N-side is connected to negative terminal

Effects:

Potential barrier decreases
Depletion region becomes thinner
Current flows easily

Once applied voltage exceeds threshold voltage (≈0.7V for silicon), current increases rapidly.

🔋 Reverse Biasing

In reverse bias:

P-side is connected to negative terminal
N-side is connected to positive terminal

Effects:

Potential barrier increases
Depletion region widens
Only a small leakage current flows

At very high reverse voltage, breakdown occurs (Zener or avalanche breakdown).

📊 I–V Characteristics of PN Junction

The I–V graph shows the relationship between current and voltage.

🔹 Forward Region:

Small current until threshold voltage
After threshold → Current increases exponentially

🔹 Reverse Region:

Very small constant current (reverse saturation current)
Sudden increase at breakdown voltage

📌 Important Points:

Diode is a unidirectional device
Conducts in forward bias
Blocks current in reverse bias
Used in rectifiers, power supplies, signal processing

🎯 By the End of This Lecture

You will:

Define PN junction and diode
Explain depletion region and potential barrier
Differentiate forward and reverse biasing
Interpret I–V characteristics of diode

This lecture builds the foundation for studying rectifiers and transistor applications in the next lessons.