Doping is when we add impurities to the extremely pure silicon. This changes the electrochemical properties of the silicon.
Silicon is in group 4 of the periodic table. Like Carbon it makes nice solid bonds in 4 directions. By adding elements from the 3rd or 5th columns we can add extra electrons or extra positive charges (holes where there are no electrons).
The elements we typically use are Boron for p-type (positive holes) doping and Phosphoros or Arsenic for n-type (extra electrons) doping.
The Arsenic is the poison in this fun tweet from @computerfact:
For more information check this article from technocrazed