Doping:
Silicon and Germanium have low values of electrical conductivity because of few free electrons. In order to improve the electrical conductivity of semi conductors' pentavelent element such as Phosphorus or Arsenic or a trivalent element Boron or Indium is added to Silicon and Germanium. This process is known as doping.
OR
The process of mixing of small quantity of Arsenic or Phosphorus in silicon or Germanium to make a n-type substance or Boron or Indium in Silicon or Germanium to make P-type substances is known as doping.