A level revision

Semiconductors, metals and insulators

Before reading this topic you should be familiar with the structure of both metals and polymers and understand why they conduct and insulate respectively. See our topics on metals and polymers to get a recap.

Semiconductors have a conductivity between that of conductors such as metals and insulators such as polymers. One way in which semiconductors and metals differ however is that as they heat up semiconductors become more conductive whilst metals become less conductive. This is due to how semiconductors are constructed.

In a semiconductor only a few of the atoms are ionised so there are fewer free electrons than in a metal.

Structure of a semiconductor: Few positive ions with few free electrons (black)

As the material is heated however the bonds between other atoms break releasing more free electrons increasing the conductivity of the material. When a metal heats up the ions and electrons vibrate more causing an increasing number of collisions between the two. As a result it becomes harder for the electrons to move through the material decreasing conductivity.

The broad difference between metals, semiconductors and insulators are the number of free electrons or 'charge carriers'

Semiconductors can be 'doped' - this process adds more ions and thus more free electrons to the material increasing conductivity.