Course: From Basic Science to Web Hosting

Module 01 — Basic Science → Electronics

What Is a Semiconductor?

Difficulty: Beginner
Prerequisites: Lesson 008 — What Is Inductance?
Estimated time: 25 minutes

1. Learning Objectives

After this lesson, you should understand:

What a semiconductor is
How conductors, insulators, and semiconductors differ
Why silicon is important
What valence electrons are
What a crystal lattice is
What energy bands mean
What a band gap is
What doping means
What P-type and N-type semiconductor materials are
Why semiconductors are the foundation of computer chips

2. The Big Transition

So far, we studied:

Matter
 ↓
Atoms
 ↓
Electrons
 ↓
Electric charge
 ↓
Electric field
 ↓
Voltage
 ↓
Current
 ↓
Circuits
 ↓
Resistance
 ↓
Capacitance
 ↓
Inductance

Now we ask:

How can we control electricity to build electronic devices?

The answer begins with semiconductors.

3. Three Important Material Categories

Materials can be broadly classified according to their electrical behavior:

Materials
   │
   ├── Conductors
   │
   ├── Semiconductors
   │
   └── Insulators

4. Conductors

A conductor allows electric charge to move relatively easily.

Example:

Copper

Simplified:

Electric field
      ↓
Mobile charge carriers
      ↓
Current

Other examples include:

Aluminum
Silver
Gold

5. Insulators

An insulator strongly restricts electrical conduction.

Examples:

Glass
Plastic
Rubber
Ceramic

Simplified:

Electric field
      ↓
Very limited charge transport

Insulators are useful because they can prevent unwanted current flow.

6. Semiconductors

A semiconductor has electrical properties between those of typical conductors and insulators, but the important feature is that its conductivity can be controlled.

A simple comparison:

Conductor
   ↓
High conductivity

Semiconductor
   ↓
Controllable conductivity

Insulator
   ↓
Very low conductivity

Common semiconductor materials include:

Silicon
Germanium
Gallium arsenide

For modern mainstream computing, silicon is especially important.

7. Why Silicon?

Silicon has atomic number:

Therefore a neutral silicon atom contains:

14 protons
14 electrons

Its electrons occupy different energy states.

For our purposes, the outermost electrons are particularly important.

8. What Are Valence Electrons?

The electrons involved in an atom’s outermost occupied shell are commonly called valence electrons.

Silicon has:

4 valence electrons

This is important because silicon atoms can form strong covalent bonds with neighboring silicon atoms.

9. Silicon Crystal

In solid silicon, atoms arrange themselves in an organized crystal structure.

Conceptually:

Si ─ Si ─ Si
│    │    │
Si ─ Si ─ Si
│    │    │
Si ─ Si ─ Si

The actual three-dimensional structure is more complex.

The atoms share electrons through covalent bonding.

10. Why Bonding Matters

In an isolated atom, electrons occupy atomic energy states.

When enormous numbers of atoms form a solid:

Atoms
 ↓
Interact
 ↓
Allowed electronic states broaden
 ↓
Energy bands

This leads to the concept of:

Energy Bands

11. What Is an Energy Band?

In a solid, electrons can occupy ranges of allowed energy rather than just the isolated-atom energy levels.

Two important bands are:

Conduction band
────────────────

Band gap
───────────────

Valence band
────────────────

The exact band structure depends on the material.

12. Valence Band

The valence band is associated with electrons involved in bonding and occupied electronic states.

For basic semiconductor physics:

Valence band
      ↓
Electrons primarily associated with bonding states

13. Conduction Band

The conduction band contains electronic states in which electrons can contribute significantly to electrical conduction.

Conceptually:

Conduction band
      ↓
Mobile conduction electrons

14. Band Gap

Between the valence and conduction bands there can be an energy range where allowed electronic states are absent.

This is the:

Band Gap

Conduction band
══════════════════

     BAND GAP

══════════════════
Valence band

The size of the band gap strongly affects the electrical properties of the material.

15. Conductor vs Semiconductor vs Insulator

A simplified picture:

Conductor

Valence/conduction states
overlap or are readily available

Therefore conduction is relatively easy.

Semiconductor

Valence band
────────────

Small/moderate band gap

────────────
Conduction band

Electrical behavior can be controlled.

Insulator

Valence band
────────────

Large band gap

────────────

Conduction band

Much less thermal excitation occurs under ordinary conditions.

16. Temperature Matters

At absolute zero in an idealized semiconductor, very few electrons have enough thermal energy to cross the band gap.

As temperature increases:

Temperature ↑
      ↓
More thermal energy
      ↓
More carriers can be excited
      ↓
Conductivity can increase

This is one reason semiconductor behavior differs from metals.

17. Light Can Also Matter

Electrons can gain energy from photons.

In suitable semiconductor materials:

Photon energy
      ↓
Electron excitation
      ↓
Electron-hole pair

This principle is used in devices such as:

Solar cells
Photodiodes
Image sensors
LEDs

18. What Is Doping?

Pure semiconductor material is called intrinsic semiconductor.

We can deliberately introduce very small concentrations of specific impurity atoms to change the semiconductor’s electrical properties.

This process is called:

Doping

Pure silicon
     ↓
Add controlled impurity atoms
     ↓
Doped silicon

This is one of the most important technologies in semiconductor manufacturing.

19. N-Type Semiconductor

Suppose silicon is doped with a suitable Group 15 donor impurity, such as phosphorus.

Phosphorus has five valence electrons.

Silicon has four.

The extra electron can contribute to conduction.

Conceptually:

Phosphorus
     ↓
Donor
     ↓
Additional conduction electron
     ↓
N-type semiconductor

The major mobile carriers are electrons.

20. P-Type Semiconductor

Now use a suitable Group 13 acceptor impurity, such as boron.

Boron has three valence electrons.

Silicon has four.

This creates an electron deficiency in the bonding structure, described as a hole.

Conceptually:

Boron
   ↓
Acceptor
   ↓
Hole
   ↓
P-type semiconductor

The major mobile carriers are holes.

21. What Is a Hole?

A hole is not a physical particle like an electron.

It is a useful model describing the absence of an electron in an otherwise occupied electronic state.

When nearby electrons move to fill that vacancy:

Electron movement
      ↓
Hole appears to move

So:

Electron = actual elementary particle

Hole = effective carrier describing missing electron

22. P-Type vs N-Type

Property	P-Type	N-Type
Dopant type	Acceptor	Donor
Majority carrier	Holes	Electrons
Example dopant	Boron	Phosphorus
Silicon remains	Semiconductor	Semiconductor

Important:

P-type does not mean the entire material has a positive electric charge.

N-type does not mean the entire material has a negative electric charge.

The material can remain electrically neutral overall.

23. Joining P-Type and N-Type

Now something very important happens.

Suppose we put:

P-type
   │
   │
N-type

together.

We have created a:

PN Junction

This is the foundation of the semiconductor diode.

24. What Happens at the Junction?

Near the boundary, electrons and holes interact and recombine.

This produces a region depleted of mobile carriers called the:

Depletion Region

Simplified:

P-type     Depletion      N-type
███████   ░░░░░░░░░░    ███████

An internal electric field develops across this region.

This creates a potential barrier that influences carrier movement.

25. PN Junction → Diode

A PN junction can be engineered into a diode.

A diode is a semiconductor device that strongly favors current in one direction under appropriate operating conditions.

Conceptually:

PN Junction
     ↓
Diode
     ↓
Controlled current behavior

We will study this in detail next.

26. Diode → Transistor

Semiconductor structures can become much more sophisticated.

PN Junction
     ↓
Diode
     ↓
Multiple semiconductor regions
     ↓
Transistor

A transistor can control current or voltage and can act as a switch or amplifier.

27. Transistor → Computer

Now the major chain appears:

Silicon
 ↓
Doping
 ↓
P-type / N-type
 ↓
PN junction
 ↓
Diode
 ↓
Transistor
 ↓
Logic gate
 ↓
Digital circuit
 ↓
Processor
 ↓
Computer

And finally:

Computer
 ↓
Operating system
 ↓
Networking
 ↓
Internet
 ↓
Web server
 ↓
Website
 ↓
Web hosting

28. Why This Lesson Is Important

This is the major transition in our course.

Before Lesson 009:

Basic physics
      ↓
Electrical circuits

After Lesson 009:

Semiconductor physics
      ↓
Electronic devices
      ↓
Computer hardware

You are now entering the technology that makes modern computers possible.

29. Quick Check