PN Junction Diode

Introduction

A PN junction is formed when a P-type semiconductor is brought into intimate contact with an N-type semiconductor. The PN junction is the fundamental building block of semiconductor electronics — diodes, transistors, solar cells, LEDs, and integrated circuits all rely on junction physics.

Formation of the PN Junction

Initial Contact

Depletion Region Formation

Process:

1. Electrons from N-side diffuse to P-side
2. Holes from P-side diffuse to N-side
3. Exposed fixed ions create an electric field
4. Electric field opposes further diffusion
5. Equilibrium reached — depletion region established

Built-in Potential (V₀)

The potential barrier that prevents further diffusion:

Potential Distribution

Depletion Width

Forward Bias

When positive terminal is connected to P-side:

Reverse Bias

When positive terminal is connected to N-side:

V-I Characteristics

Diode Equation

Numerical Examples

Example 1: Built-in Potential

Problem: A silicon PN junction has Na = 10¹⁷/cm³ and Nd = 10¹⁵/cm³. Calculate the built-in potential at 300K.

Solution:

Example 2: Forward Current

Problem: A silicon diode has Is = 10 nA. Find the forward current at V = 0.65V and T = 300K (n = 1).

Solution:

Example 3: Depletion Width

Problem: For the junction in Example 1, find the depletion width with no external bias. εᵣ(Si) = 11.7.

Solution:

Junction Capacitance

Depletion Capacitance (Reverse Bias)

Diffusion Capacitance (Forward Bias)

Interview Questions

1. Why does a PN junction have a built-in potential but no current flows in equilibrium?

The built-in potential creates an electric field that exactly balances the diffusion tendency. Drift current (due to field) equals and opposes diffusion current for both carriers. Net current is zero. No external voltage appears across terminals due to contact potentials.

1. What determines the width of the depletion region?

Depletion width depends on: doping concentrations (lower doping → wider), applied bias (reverse bias widens, forward narrows), and built-in potential. The depletion region extends more into the lightly-doped side.

1. Explain why forward current is exponential while reverse current is nearly constant.

In forward bias, barrier reduction exponentially increases the number of carriers with enough energy to cross (Boltzmann statistics). In reverse bias, current is limited to thermally-generated minority carriers in the depletion region, which is temperature-dependent but voltage-independent.

1. What is the difference between depletion and diffusion capacitance?

Depletion capacitance arises from charge storage in the depletion region (like a parallel plate capacitor), dominant in reverse bias. Diffusion capacitance arises from minority carrier charge stored near the junction during forward conduction, dominant in forward bias.

1. How does temperature affect PN junction behavior?

Temperature increases nᵢ exponentially, increasing Is (doubles every ~10°C). Forward voltage drops ~2 mV/°C for constant current. Breakdown voltage increases for avalanche mechanism, decreases for Zener mechanism.

Summary

The PN junction is the cornerstone of semiconductor electronics. Understanding depletion region formation, built-in potential, forward/reverse bias behavior, and the diode equation is essential for analyzing all semiconductor devices. The junction's non-linear V-I characteristic enables rectification, switching, and signal processing functions fundamental to analog electronics.