Any impurity added to a semiconductor to modify its electrical conductivity. The most common semiconductors, silicon and germanium, form crystalline lattices in which each atom shares electrons with four neighbours (see bonding). Replacing some atoms with donor atoms (e.g., phosphorus, arsenic) that have five bonding electrons makes extra electrons available. The semiconductor thus doped is called n-type (for negative, because of the additional negative charges). Doping with acceptor atoms (e.g., gallium), which have only three electrons available, creates holes, which are positively charged. Conduction can occur by migration of holes through the crystal structure of such a semiconductor, known as p-type (for positive).
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