Wikipedia article: http://en.wikipedia.org/wiki/Doping_(semiconductor [Broken])
Pure semiconductors (often cut into flat circular disks from a large single crystal [called a boule] grown into a shape that looks like the inside of your coffee thermos) have a very regular arrangement of nuclei with very predictable electonic properties, due to their (almost) perfect crystalline structure. You can add small (controlled) amounts of a second semi-conductor to increase the number of charge carriers available.
Remember, an insulator does not conduct electricity, because there are no/few free charge carriers. A conductor does, because of the large number of available free charge carriers. A semiconductor is somewhere in between. You can control the conductivity of the semi-conductor by controling the number of free charge carriers - and this is done by controlling the type and amount of material used for doping.
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