To obtain a p-type extrinsic semiconductor, an intrinsic semiconductor like Germanium (a Group 14 element with 4 valence electrons) must be doped with a trivalent impurity (an element from Group 13 with 3 valence electrons). Boron is a Group 13 element (trivalent) and its addition creates holes in the crystal lattice, resulting in a p-type semiconductor. Conversely, antimony, phosphorous, and arsenic are all pentavalent impurities (Group 15) and would create an n-type semiconductor by contributing extra electrons.