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).

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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