conductivity of silicon increases on doping it with phosphorus reason

Phosphorus doping introduces phosphorus atoms in silicon crystal leading to change it's electrical properties. Phosphorus atoms have 5 valence electrons which causes n-type doping. A phosphorus atom replaces silicon atom(having four valence electrons) in silicon crystal. Four out of five valence electrons of phosphorus take part in bonding while the fifth electron remain free. When numerous phosphorus atoms are substituted for silicon in crystal, many electrons are free which lead to increase in conductivity of silicon crystal.