Q.How many atoms are present per unit cell of a body centred cubic crystal?

Semiconductors (n-type and p-type)

Semiconductors are solids whose electrical conductivity lies between that of metals and insulators, typically $10^{-6}$ to $10^{4}\ \Omega^{-1}\text{m}^{-1}$, and it increases with temperature. Pure silicon and germanium are intrinsic semiconductors: at 0 K they behave as insulators, but on heating a few electrons cross the small band gap into the conduction band, leaving positively charged "holes" in the valence band. Both the electrons and the holes carry current.

Conductivity is greatly enhanced by doping — adding tiny amounts of an impurity element — giving extrinsic semiconductors of two kinds.

n-type: Silicon (group 14, four valence electrons) is doped with a group-15 element such as phosphorus or arsenic, which has five valence electrons. Four electrons form bonds with silicon; the fifth is delocalised and free to conduct. The majority charge carriers are negative electrons, hence "n-type." …