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A semiconductor's conductivity can be greatly increased by adding very small amounts of other elemments to the pure (intrinsic) material. The process of doing this is called doping. Doping adds impurity atoms to the intrinsic material in such a way that they fit into the original crystalline pattern, but in doing so they increase either the number of free electrons or the number of holes (but not both). This reduces the resistivity to a relatively low value.

If the doped semiconductor has an excess of free electrons it is called n-type material; if it has an excess of holes it is called p-type. Despite these excess of one type or the other of carriers, the material remains electrically neutral. This can be understood on the basis that the impurity atoms are themselves electrically neutral and therefore do not upset the neutrality of the intrinsic semiconductor.

Doped material is capable of conducting equally well in any direction, and does not differ from intrinsic material in this respect. The resistivity of the doped semiconductor depends on the amount of inpurity added; light doping results in relatively high resistivity and heavy doping results in a relatively low value.