Doh! What is doping? Why do we "dope" semiconductors?

Doping is when you intentionally add impurities to a material. Why, then, after going to the trouble to purify the material do you purposely undo your good work? Dopants change the electronic properties of a material. In any material, you can only have electrons at certain energies. These energies, in condensed matter (liquids and solids), are typically found in bands, or ranges, of allowed energies.

In a semiconductor, you have a gap between energy states that likely have electrons in them (valence band) and energy states that likely don't have electrons in them (conduction band). Thermal energy makes it so that there is a likelihood (relatively small) of finding an electron in the conduction band although, if all states were filled from the lowest energy on up, there would be no electrons in the conduction band. Dopants add or remove electrons from the mix, either increasing or decreasing the likelihood of finding electrons in the conduction band. P-type dopants make there be fewer electrons than a semiconductor (like silicon) without these dopants. N-type dopants make there be more electrons than a semiconductor without these dopants. The electrons in the conduction band and the missing charges, or holes, in the valence band allow charge to flow when a voltage (electric potential) is applied. Check out this interactive demonstration to see how doping changes carrier concentrations and allows charge to flow! If you have difficulty viewing this, download this software (http://www.wolfram.com/cdf-player/).

Doped Silicon Semiconductors from the Wolfram Demonstrations Project by S. M. Blinder

The energy above and below which you are equally likely to find an electron at temperatures above absolute 0 K is called the Fermi level. This can be found within the energy gap or within a band although, in semiconductors, it is typically somewhere in the gap.