When silcon is doped with some group-15 element, the some of the positions in the lattice are substituted by atoms fo groups -15 elements have five valence electrons. After forming the  four covalent bonds with silicon (or anyother group-14 element such as germanium). one excess electron is left on them. Since this electron is not involed in bonding it becomes delocalized and contributre to electrical conduction. Silicon dped with group 15 element behaves as a n-type semconductor.