Conductors, Semiconductors & Insulators (OCR AS Physics)

• 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