A material, typically crystaline, which allows current to flow under certain circumstances. Common semiconductors are silicon, germanium, gallium arsenide. Semiconductors are used to make diodes, transistors and other basic "solid state" electronic components.
As crystals of these materials are grown, they are "doped" with traces of other elements called donors or acceptors to make regions which are n- or p-type respectively for the electron model or p- or n-type under the hole model. Where n and p type regions adjoin, a junction is formed which will pass current in one direction (from p to n) but not the other, giving a diode.
One model of semiconductor behaviour describes the doping elements as having either free electrons or holes dangling at the points in the crystal lattice where the doping elements replace one of the atoms of the foundation material. When external electrons are applied to n-type material (which already has free electrons present) the repulsive force of like charges causes the free electrons to migrate toward the junction, where they are attracted to the holes in the p-type material. Thus the junction conducts current.
In contrast, when external electrons are applied to p-type material, the attraction of unlike charges causes the holes to migrate away from the junction and toward the source of external electrons. The junction thus becomes "depleted" of its charge carriers and is non-conducting.