Conductors, Semiconductors & Insulators

The number density of charge carriers, n of a material determines how well the material conducts electricity

Value of n For Different Materials Table

Conductors have a very large value of n
- This is often in the range of 10²⁸ m^-3
This means there are many free electrons per unit volume moving through the material
Since current is the rate of flow of electrons, this makes them very good electrical conductors
Examples of good electrical conductors are copper, aluminium and calcium
- This is why electric wires are often made from copper and aluminium

Semiconductors are materials that have a conductivity between conductors and insulators
- Their conductivity depends on their temperature
- At low temperatures, their resistivity rises, therefore their conductivity falls
- At high temperatures, their resistivity falls, therefore their conductivity rises
- Metal conductors behave in the opposite way
Examples of semiconductor materials are silicon and germanium
They are often used to make electronic devices such as diodes and transistors
Semiconductors have very intermediate conduction properties
- This means they are neither very good nor very poor in terms of their conducting properties
Therefore, semiconductor materials undergo doping
- This is when impurities are added to the material
- This helps improve their conducting properties, by increasing the value of n

Insulators have a very small value of n
- This is close to 0
This means there are next to no free electrons per unit volume moving through the material
With no current, this makes them very poor electrical conductors
Examples of electrical insulators include plastic, rubber and glass
- These are useful for making parts of an object in which heat or electricity should not to flow for safety reasons eg. the casing of a plug

Conductors, Semiconductors & Insulators (OCR A Level Physics)