Pentavalent elements are those elements which have five electrons in their outer shell. To make the n-type semiconductor, pentavalent impurities like phosphorus or arsenic are added. Four of the impurities' electrons form bonds with the surrounding silicon atoms. This leaves one electron free. The resulting material has a large number of free electrons. Since electrons are negative charge carriers, the resultant material is called an n-type (or negative type) semiconductor. The pentavalent impurity that is added is called a 'dopant' and the process of addition is called 'doping'.