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A quadrivalent element (like Silicon or Germanium) has four valence electrons. When it is doped with a pentavalent element (like Phosphorus, Arsenic, or Antimony), the pentavalent impurity atom replaces an atom of the quadrivalent element in the crystal lattice. The pentavalent atom uses four of its valence electrons to form covalent bonds with the surrounding atoms, leaving one extra electron that is loosely bound.This extra electron can easily move into the conduction band, increasing the number of free electrons (which are negative charge carriers). This type of semiconductor, where electrons are the majority carriers, is called an N-type semiconductor.