Conductors, Semiconductors & Insulators (OCR AS Physics): Revision Note

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

• 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

• 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

• 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