Why is silicon doped ?

Silicon in its pure crytalline state is an insulator. This is because Silicon with 4 electrons in its outermost shell (With atomic no. 14) combines with 4 other silicon atoms sharing one electron each with them, hence becoming stable in the process. So pure silicon doesnt have any free electrons. If given some external energy, some electrons will break free, but still it will still be more or less an insulator.
So silicon is doped with an impurity like phosphorous having 5 electrons in its outer shell. 4 electrons will combine with 4 silicon atoms, but still one phosphorous electron will be free. Hence doping phosphorous with Si greatly improves its conductivity. (Despite the fact that quantity of Phosphorous will be very minute, maybe 1 in 1000.). This type of semiconductor is called as n type semiconductor.