Conductors pass electricity with little resistance. All metals are conductors, with a typical resistivity of around
ohm metres. A material may only be a good conductor of electricity if the material contains free electrons, able to move freely, since it is electrons that carry electric current.
Insulators do not conduct electricity well. They have very high resistances – typically 1000 %ohm metres and higher. They do not contain free electrons able to conduct electricity. Examples include wood, polystyrene, polymers.
Semiconductors are a intermediate between these two. The resistivity is of the order of
ohm metres.
Many conductors obey ohm's law for certain ranges of temperature, so that the current passing through is proportional to the voltage across the resistor and the resistance is constant. Outside this range though, this is not true. If the temperature rises, the resistance increases and if the temperature falls, so does the resistance. This is because an increasing temperature gives vibrational kinetic energy to the electrons. They can then impede the passage of the electrons that carry the current so the resistance increases.
Insulators work the other way. They contain very few free electrons. In order to create some, the electrons must gain energy. One way to do this is to heat the material. The electrons can move to higher energy levels in which they can move in response to an applied voltage and so carry a current.
Semiconductors – the best known being silicon and germanium - are intermediate between conductors and insulators. The electrical properties can be influenced significantly by the addition of small amounts of impurities – a process called doping. Like insulators, the resistance can be reduced by heating, but the amount of heating required is much less. Semiconductors can be used to make logic devices – transistors – that are at the heart of electronics.