Crystalline solids exhibit an important property called conductivity. Conductivity is the property of a material by which it allows the flow of electric current. Conducting materials are generally classified into three types namely conductors, semi conductors and super conductors.
Typical metals are good conductors of electricity while elements like silicon and germanium are non conductors at ordinary temperature. However, they exhibit appreciable conductivity upon addition of impurities as Arsenic and Boron. The resulting materials are called semiconductors. Semi conductors which exhibit conductivity due to the flow of excess negative electrons are called n-type semiconductors. The conductivity due to the positive holes are called p-type semiconductors. Semiconductors find application in modern devices as recitifiers, transitors and solar cells.