When a pentavalent impurity (Valency 5) atom is added pure Si or Ge, four of the five valence electrons of the impurity atom form covalent bonds with one each valence electrons of four Ge or Si atoms surrounding it; while the fifth remains very weakly bound to the parent atom. As a result inonisation energy required to set this electron free is very small and even at room temperature it will be free to move about in the crystal. [For example, the energy required is 0.01 eV for Ge and 0.05 eV for Si]. Thus, the pentravalent dopant is donating one extra electron for conduction and hence is konwn as donor impurity.
In a doped semiconductor the total number of conduction electrons ne is due to electrons contributed by donors and those generated intrinsically, while the total number of holes nh , is only due to the holes intrinscially. Hence electrons become the majority charge carriers and holes the minority carriers. These semiconductros are therefore known as n–type semiconductors. For n–type semiconductor, we have
ne >> nh
