A  conductor is a material that allows the flow of energy or particles, such as electric charge or heat. If a material prevents the flow of electric current and does not conduct, it is an insulator.

Insulators

In an insulator, electrons are tightly bound to atoms and cannot move freely throughout the material. This is because:

• Charge carriers (electrons or ions) are strongly attached to their respective atoms, preventing conduction
• The material has a high specific resistance, meaning it strongly opposes the flow of electricity  by frequent collisions with other atoms or nuclei

Example materials: rubber, glass, plastic…

Conductors

A conductor is a material with freely moving charge carriers, making it capable of transporting electrical current.

Conductors have low specific resistance, allowing charge to flow easily.

The valence electrons (electrons in outermost shell of atom) are loosely bound and can move freely. Many conductors, such as metals, form a crystal lattice structure, where electrons are delocalized (not bound to a single atom).

The best electrical conductor is silver.

Types of conductors

1.         First-class conductors (metallic conductors): metals such as copper, silver, and aluminium, where free electrons move within a lattice.

2.         Second-class conductors (ion conductors): substances that conduct via ions, rather than free electrons. This process occurs in electrolytes.

The conductivity of ion conductors arises from the dissociation (splitting) molecules into charged ions in an electrolyte.

Salt solutions, for example, separate into positive and negative ions, which cause conductivity. The positive ions move toward the negative cathode, and the negative ions move toward the positive anode, enabling current to flow.

Conductivity and quantum mechanics

The electrical conductivity of a material is deeply connected to quantum mechanics.

Delocalized electrons cause electrical conductivity. These electrons do not have fixed positions and can move freely, we cannot know their exact position.

The Pauli principle stated that two electrons cannot occupy the same quantum state. If all available energy states are filled, additional electrons cannot move, preventing conductivity.

High conductivity occurs in materials with many partially occupied energy states, allowing electrons to move between them.  

Semiconductors

A semiconductor has electrical conductivity between that of a conductor and an insulator. Their conductivity depends on many factors such as temperature or material.

In their pure state, semiconductors do not conduct electricity well. This atomic lattice is perfectly structured, neither free electrons nor holes exist.

Example: silicon has four valence electrons, but seeking eight (octet rule). In the lattice, the silicon atoms share the electrons so that each atom has exactly eight valence electrons around it. This stable state allows no free electrons for conduction.

To make a semiconductor conduct, we need to create charge carriers. This can be done by the temperature or pressure releases electrons.

 Also, we can introduce impurities to modify the electron structure- this is called doping.

Doping

Doping is the process of  contaminating a semiconductor with other substances to increase its charge carriers.

1. N-Doping (negative doping)

• A substance with more valence electrons id added (e.g., phosphorus, which has five valence electrons).
• This creates additional free electrons, allowing current to flow.
• The material has a net negative charge due to the additional electrons.

     2.   P-Doping (positive doping)

• A substance with fewer valence electrons is added (e.g., boron, which has three valence electrons).
• This creates electron vacancies, known as holes, which act as positive charge carriers.
• The material has a net positive charge due to the middling electrons

By combining n-doped and p-doped regions in a single semiconductor, we can create p-n junctions, the foundation of modern electronics.

Superconductors

In most materials, electricity flows with some resistance, leading to energy loss. However, in superconductors, electricity flows without resistance, meaning zero energy loss. This makes the extremely valuable for practical applications.

How do superconductors work?

Superconductivity is based on the formation Cooper Pairs – pairs of electrons that move in sync.

Normally, electrons experience resistance as they scatter off vibrating atoms. In superconductors, at extremely low temperatures, electrons form Cooper pairs, which move without scattering.

These pairs behave like a single quantum entity, moving through the lattice without resistance.

The cooler the temperature gets, the more organized the pathways of the Cooper pairs and the nuclei get.

Challenges

Superconductors require extremely low temperatures, making practical application difficult. However, they are used in MRI machines, particle accelerators, and maglev trains.