Silicon and germanium both have valance electrons and they belong to group 14^th of the periodic table. Arsenic and phosphorus belong to group 15^th of the periodic table and they have valence electrons equal to 5. When silicon or germanium is adopted with phosphorus or arsenic, four electrons of phosphorus or arsenic out of five; make covalent bonds with four electrons of silicon or germanium leaving one electron free; which increases the electrical conductivity of silicon or germanium. Since the electrical conductivity of silicon and phosphorus is increased because of negatively charged particles (electron), thus this is known as n – type semiconductor.