Engineering Physics

8.4 Extrinsic semiconductors

The conductivity of an intrinsic semiconductor can be increased enormously by adding small amounts of impurity atoms [such as III or V group atoms]. For example, the conductivity of silica is increased by 1000 times on adding 10 parts of boron per million part of Si. The process of adding impurities is called doping and the impurity added is called dopant. The doping is done during crystallization process [i.e., when the substance is in molten state]. On crystallization, the impurity atoms replace some of the intrinsic atoms.

When some pentavalent [Group V] impurity atoms such as P [or As, Sb, Bi, etc.] are added to an intrinsic semiconductor Si (or Ge), then the impurity atoms interlock in the crystal lattice because ...