0• Pentavalent impurities such as phosphorus, arsenic, antimony, and bismuth have 5 valence electrons.
• When phosphorus impurity is added to Si, every phosphorus atom’s four valence electrons are locked up in covalent bond with valence electrons of four neighboring Si atoms. However, the 5th valence electron of phosphorus atom does not find a binding electron and thus remains free to float. When a voltage is applied across the silicon-phosphorus mixture, free electrons migrate toward the positive voltage end.
• When phosphorus is added to Si to yield the above effect, we say that Si is doped with phosphorus. The resulting mixture is called N-type silicon (N: negative charge carrier silicon).
• The pentavalent impurities are referred to as donor impurities
