Chapter 2

Introduction

In the previous chapter, we discovered that every material is built from atoms, and that the outermost electrons largely determine how a material behaves.

But if all matter is made from atoms, some obvious questions arise.

Why does electricity flow easily through metals but not through rubber? Why can a metal spoon conduct electricity, while the plastic handle remains completely safe to touch? And why is silicon, the material found inside almost every solar panel, different from both?

The answer lies in how neighbouring atoms interact with one another inside a solid. In some materials, these interactions allow electrons to move freely. In others, they keep electrons firmly attached to their atoms. A small group of materials sits somewhere in between, and it is this balance that makes solar cells possible.

Conductors: Electrons Free to Move

In some materials, the outer electrons are held only loosely by their atoms. When individual atoms come together to form a solid, their outer electrons begin interacting with those of neighbouring atoms. In metals, these interactions are so strong that many of the outermost electrons are no longer tied to a single atom. 

Instead, they become shared throughout the entire material, forming what is often described as a sea of electrons.

Rather than belonging to one atom, these electrons are free to move almost anywhere within the metal. They behave much like people walking through an open public square, able to travel in any direction without being confined to a single location.

Normally, these electrons move randomly in all directions. However, when a voltage is applied, they begin drifting together in the same direction. This organised movement of electrons is what we call an electric current.

Materials that behave in this way are known as conductors.

Metals such as copper, aluminium and silver are excellent conductors because they contain large numbers of these freely moving electrons. This is why electrical wiring, power cables and many electronic components are made from these materials.

Without conductors, it would be almost impossible to transport electricity from one place to another.

Insulators: Electrons Locked in Place

Insulators behave in almost the opposite way. In substances such as rubber, glass and plastic, neighbouring atoms hold tightly onto their outer electrons. Instead of becoming shared throughout the material, the electrons remain closely associated with their own atoms and cannot move freely from one atom to the next.

Even when a voltage is applied, these electrons remain firmly in place. Without freely moving electrons, an electric current cannot flow. These materials are known as insulators because they resist the movement of electricity.

This property makes them just as important as conductors. Plastic coatings around electrical cables, rubber handles on tools and ceramic components inside electrical equipment all help prevent electricity from travelling where it shouldn't. Good insulators make electricity safer by keeping it confined to the conductors designed to carry it.

Semiconductors: The Best of Both Worlds

Between conductors and insulators lies a remarkable class of materials known as semiconductors. Their electrons are neither completely free nor permanently trapped. Instead, they occupy a unique middle ground.

Under normal conditions, very little electricity flows through a semiconductor because most of its outer electrons remain attached to their atoms. However, unlike an insulator, they are not held quite as tightly. If enough energy is supplied, some of these electrons can break free and begin moving through the material. This means a semiconductor can behave like an insulator in one situation and more like a conductor in another.

This ability to control the flow of electricity makes semiconductors unlike any other class of material.

Modern technology depends on them. Smartphones, computers, LEDs, microprocessors, electric vehicles and solar panels all rely on semiconductors to carefully control the movement of electrons. Without semiconductors, the modern electronic world simply would not exist.

Why Silicon?

Among all semiconductor materials, one stands above the rest. Silicon.

Silicon is the second most abundant element in Earth's crust, making it inexpensive, widely available and relatively easy to manufacture on a large scale. 

More importantly, silicon holds onto its outer electrons with almost the perfect amount of strength. If they were held any more loosely, silicon would behave like a conductor and electricity would flow continuously, making it impossible to control. If they were held any more tightly, sunlight would struggle to release them at all, making the material ineffective for generating electricity. Instead, silicon occupies the ideal middle ground.

The energy carried by ordinary sunlight is almost perfectly matched to the amount of energy needed to free many of silicon's outer electrons. This unique balance allows a solar cell to convert light into electricity efficiently while remaining stable and reliable for decades.

For this reason, more than 90% of the world's solar panels are made from silicon.

Looking Ahead

So far, we have explained electrical behaviour by describing how tightly different materials hold onto their outer electrons. This picture is useful, but it is only part of the story.

The real explanation lies much deeper inside the material itself.

Electrons inside a solid cannot possess just any amount of energy they like. Instead, nature only allows certain ranges of energy, while other energies are completely forbidden.

These allowed and forbidden energy regions determine whether electrons can move freely, remain trapped or be released when energy, such as sunlight, is supplied. They explain why metals are conductors, why glass is an insulator, and why silicon sits perfectly between the two.

But why does nature place these restrictions on electrons in the first place?

The answer lies in a hidden world that exists inside every solid.

In the next chapter, we'll explore energy bands and discover why electrons can only occupy certain energies, and how this simple idea explains the behaviour of every conductor, insulator and semiconductor.