semiconductor
    n 1: a substance as germanium or silicon whose electrical
         conductivity is intermediate between that of a metal and an
         insulator; its conductivity increases with temperature and
         in the presence of impurities [syn: semiconductor,
         semiconducting material]
    2: a conductor made with semiconducting material [syn:
       semiconductor device, semiconductor unit,
       semiconductor]

	

	semiconductor

    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.

   (1995-10-04)