What is a semiconductor?

"Monokristalines Silizium für die Waferherstellung". Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons

A semiconductor is a material that typically conducts electricity better than an insulator, but not as good as a metal. Electrons in the material can move around, but, in order to move around, they must overcome an energy barrier. Things at higher temperatures have more energy. Some electrons in a semiconductor are able to overcome the energy barrier (between what are known as the valence and conduction bands) and move. Dopants (other added elements) can be added to the material to change the electron concentration. Places where an electron is absent but is of low enough energy that an electron "should" be there are known as holes. It is these mobile electrons and mobile holes (absences of electrons) that conduct electricity. Semiconductors are widely used to make solar cells as well as computer chips. They are extremely useful materials.