Question

What happens when phrosphorus is added to pur silicon

Answer 1

N-type semiconductor is formed when in the regular crystal lattice structure of Silicon, atoms of group 15 elements like phosphorous are introduced (By the process of doping). Due to presence of 5 electrons in the valence shell of phosphorous, 4 electrons are consumed in forming covalent bonds leaving behind 1 electron which remains free to conduct electricity. 

Answer 2

it forms a n type extrinsic semiconductor
since pure silicon is a semiconductor