The function of a donor in extrinsic semi-conductor is to:

CONCEPT:

• Doping: The process of adding impurities to a pure (intrinsic) semiconductor is called doping.
  • By doing this, the conductivity of the semiconductor is increased drastically even at room temperatures.
  • It is needed because the conductivity of a pure semiconductor like silicon or germanium is very low at room temperatures.
  • Doping leads to the formation of extrinsic semiconductors which are of two types: n-type and p-type.
• N-type semiconductors have impurities of elements from Group 5 of the periodic table, hence electrons are the majority charge carriers.
• P-type semiconductors have impurities of elements from Group 3 of the periodic table, hence holes are the majority charge carriers.

​EXPLANATION:

• The elements whose atom has five valance electrons are called pentavalent impurities e.g. As, P, Sb, etc.
• These impurities are also called donor impurities because they donate an extra free electron.  
• Thus, the n-type semiconductor is obtained if we dope Silicon with a pentavalent element.