Why can a tiny amount of impurity greatly increase silicon’s conductivity?

In its pure (intrinsic) state, silicon is a poor conductor of electricity because its electrons are tightly bound in covalent bonds within its crystal lattice. To increase its conductivity, a process called doping is used, which involves intentionally adding a tiny amount of specific impurities to the silicon.12

The increase in conductivity occurs because these dopant atoms alter the number of available charge carriers:

• n-type doping: When silicon is doped with elements like phosphorus or arsenic, which have five valence electrons, four electrons form bonds with neighboring silicon atoms, while the fifth electron is left free to move through the lattice, acting as a charge carrier.213
• p-type doping: When silicon is doped with elements like boron, which have only three valence electrons, the resulting bond deficiency creates a positively charged site known as a hole. This hole can migrate through the crystal lattice, effectively acting as a positive charge carrier.21

By introducing these extra electrons or holes, the material becomes an extrinsic semiconductor with significantly higher electrical conductivity than pure silicon.1