What is a semiconductor?

Semiconductors are a really cool group of materials. They are critical in allowing your computer to work. They are also found in technologies like lasers, diodes, televisions, and most microchips.

A material is made of atoms. Those atoms contain electrons. Because of quantum mechanics, those electrons can only have certain well-defined energies. Some of these energies in a material are very close together (have similar energies). Some of these energies in a material have a significant gap in between them of more than 0.5 eV (a unit of energy measure). Not all of the energy states allowed in a material actually have electrons in them. In fact, the electrons like to be in the lowest energy state. Energy in the environment (around 0.026 eV), though, allows some electrons to be of higher energy, although you still find that most electrons are in the lower energy states. Something called the Fermi level describes the point to which the energy states are filled. It is a sort of potential energy of the electrons. As you can see in the image below, in a semiconductor or insulator, the Fermi level is located below the conduction band (a grouping of largely unoccupied electron states) and above the valence band (the highest energy grouping of occupied electron states). In a semiconductor, however, the gap, called the band gap, is small enough that you can randomly get electrons in the conduction band states at room temperature.

Like the name implies, the electrons in the conduction band can more easily move between atoms than electrons in the valence band. The place where the electrons came from in the valence band (an absence of an electron) is called a hole. These holes can also move and conduct electricity. In a metal, since there is no gap between the "conduction" and "valence" bands (they are in the same band), electrons can already freely move and electrons conduct fairly freely between atoms.