Types of Semiconductors in Solid State Physics

 

Introduction

Semiconductors are one of the most important classes of materials in solid state physics. They are the foundation of modern electronic devices such as transistors, diodes, integrated circuits, solar cells, LEDs, sensors, and microprocessors. A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Conductors like copper allow electric current to flow easily, while insulators like rubber strongly resist current. Semiconductors possess intermediate conductivity, and their conductivity can be controlled by temperature, impurities, electric field, and light.

The study of semiconductors is important because almost every electronic device used today depends on semiconductor materials. Silicon and germanium are the most commonly used elemental semiconductors. In addition, compound semiconductors such as gallium arsenide and cadmium sulfide are also widely used.

Semiconductors are mainly classified into different types based on purity and doping. In solid state physics, the two major types are:

1. Intrinsic Semiconductor

2. Extrinsic Semiconductor

Extrinsic semiconductors are further divided into:

n-type semiconductor

 p-type semiconductor

 What is a Semiconductor?

A semiconductor is a material having four valence electrons in its outermost shell. These valence electrons form covalent bonds with neighboring atoms in the crystal lattice. At absolute zero temperature, semiconductors behave like insulators because all electrons are bound in covalent bonds. However, when temperature increases, some electrons gain enough thermal energy to break free from the bonds and move into the conduction band. This creates free electrons and holes, allowing current flow.

Thus semiconductors show properties between metals and insulators.

Examples:

•  Silicon (Si)
•  Germanium (Ge)
•  Gallium Arsenide (GaAs)
•  Indium Phosphide (InP)

1. Intrinsic Semiconductor

An intrinsic semiconductor is a pure semiconductor without any impurity atoms added. It contains only atoms of the semiconductor material itself.

Examples:

•  Pure Silicon
• Pure Germanium

In intrinsic semiconductors, the number of free electrons is equal to the number of holes. When thermal energy is supplied, some covalent bonds break, releasing electrons into the conduction band. The vacant positions left behind are called holes.

Therefore:

• Number of electrons = Number of holes

This means both electrons and holes take part in electrical conduction.

 Properties of Intrinsic Semiconductor

1. Pure form of semiconductor

2. No impurities added

3. Electrons and holes are equal in number

4. Low conductivity at room temperature

5. Conductivity increases with temperature

6. Behaves as insulator at very low temperature

 Energy Band Theory

In intrinsic semiconductor:

•  Valence band is almost full
•  Conduction band is almost empty

 A small forbidden energy gap exists between them

For silicon, band gap ≈ 1.1 e V

For germanium, band gap ≈ 0.7 e V

Because the band gap is small, electrons can jump from valence band to conduction band at room temperature.

 Applications

Intrinsic semiconductors are mainly used for understanding semiconductor theory and in some special devices.

2. Extrinsic Semiconductor

An extrinsic semiconductor is obtained by adding a very small amount of impurity atoms to a pure semiconductor. This process is called doping.

Doping greatly increases conductivity.

Usually, one impurity atom is added per million semiconductor atoms.

Extrinsic semiconductors are of two types:

1. n-type semiconductor

2. p-type semiconductor

(a) n-type Semiconductor

An n-type semiconductor is formed when a pentavalent impurity is added to pure silicon or germanium.

Pentavalent atoms have five valence electrons.

Examples:

• Phosphorus (P)
•  Arsenic (As)
•  Antimony (Sb)

When phosphorus is added to silicon:

 Four valence electrons form covalent bonds with neighboring silicon atoms

 The fifth electron is loosely bound and becomes free

Thus one free electron is donated by each impurity atom.

Because electrons are negatively charged, it is called n-type semiconductor.

 Charge Carriers

•  Majority carriers = Electrons
•  Minority carriers = Holes

 Properties of n-type Semiconductor

1. High conductivity compared to intrinsic semiconductor

2. Current mainly due to electrons

3. Electrons are majority carriers

4. Holes are minority carriers

5. Fermi level lies close to conduction band

 Energy Band Diagram

A donor energy level appears just below the conduction band. Electrons can easily move into conduction band with little energy.

 Uses

•  Transistors
•  Rectifiers
•  IC circuits
• Electronic switches

 (b) p-type Semiconductor

A p-type semiconductor is formed by adding a trivalent impurity to pure silicon or germanium.

Trivalent atoms have three valence electrons.

Examples:

•  Boron (B)
•  Aluminium (Al)
•  Gallium (Ga)
•  Indium (In)

When boron is added to silicon:

Three electrons form covalent bonds

 One bond remains incomplete

This missing electron creates a hole.

Since holes behave like positive charge carriers, it is called p-type semiconductor.

 Charge Carriers

 Majority carriers = Holes

 Minority carriers = Electrons

 Properties of p-type Semiconductor

1. Conductivity higher than intrinsic semiconductor

2. Current mainly due to holes

3. Holes are majority carriers

4. Electrons are minority carriers

5. Fermi level lies close to valence band

 Energy Band Diagram

An acceptor level appears just above the valence band. Electrons can move from valence band to acceptor level, leaving holes in valence band.

 Uses

• Diodes
•  Transistors
•  Sensors
•  Integrated circuits

 Difference Between n-type and p-type Semiconductor

Property	n-type	p-type
Impurity added	Pentavalent	Trivalent
Examples	P, As, Sb	B, Al, Ga
Majority carriers	Electrons	Holes
Minority carriers	Holes	Electrons
Fermi level	Near conduction band	Near valence band

 

 

 Difference Between Intrinsic and Extrinsic Semiconductor

 

Property	Intrinsic	Extrinsic
Purity	Pure	Impure (doped)
Conductivity	Low	High
Charge carriers	Equal electrons & holes	Unequal
Uses	Basic theory	Electronic devices

 

 Compound Semiconductors

Apart from silicon and germanium, compound semiconductors are also important.

Examples:

• Gallium Arsenide (GaAs)
•  Cadmium Sulfide (CdS)
• Indium Phosphide (InP)
• These are used in:
•  High-speed devices
•  Microwave devices
•  LEDs
•  Laser diodes
•  Solar cells

Gallium arsenide has high electron mobility, making it useful in fast circuits.

 Importance of Doping

Doping is extremely important in semiconductor technology. By controlling impurity concentration, conductivity can be precisely adjusted.

Advantages:

1. Increases conductivity

2. Creates p-n junctions

3. Enables transistor action

4. Used in microchips

5. Improves efficiency of devices

Without doping, modern electronics would not exist.

Semiconductor in Daily Life

Semiconductors are used in:

•  Mobile phones
•  Computers
•  Television
•  LED bulbs
• Solar panels
• Medical instruments
•  Automobiles
•  Internet devices

Every modern smart device contains semiconductor components.

 Why Silicon is Most Commonly Used?

Silicon is preferred because:

1. Easily available from sand

2. Low cost

3. Stable at high temperature

4. Forms strong oxide layer (SiO₂)

5. Suitable band gap

This is why most IC chips are made of silicon.

Conclusion

Semiconductors are materials with conductivity between conductors and insulators. In solid state physics, they are mainly classified into intrinsic and extrinsic semiconductors. Intrinsic semiconductors are pure materials such as silicon and germanium. Extrinsic semiconductors are produced by doping and are of two types: n-type and p-type. In n-type materials, electrons are majority carriers, while in p-type materials, holes are majority carriers.

Semiconductors are the backbone of modern electronics. Their controlled conductivity makes them useful in diodes, transistors, solar cells, sensors, and integrated circuits. Understanding the types of semiconductors is essential for studying solid state physics and electronic engineering.