How can tiny amounts of impurities change a semiconductor’s electrical conductivity?

In pure semiconductors like silicon, atoms are held together by covalent bonds where each atom shares its four outer electrons with neighbors. Because these electrons are tightly bound, pure silicon is a poor conductor at room temperature.12

Electrical conductivity is significantly increased through doping, a process of intentionally introducing tiny amounts of specific impurities into the crystal lattice. The effect depends on the atomic structure of the dopant:31

• n-type semiconductors: When atoms with five outer electrons (such as phosphorus or arsenic) replace silicon atoms, four electrons form bonds, leaving the fifth electron free to move through the crystal. These "donor" atoms provide extra negative charge carriers.312
• p-type semiconductors: When atoms with only three outer electrons (such as boron) are used, they create a vacancy in the bonding structure known as a hole. This hole acts as a mobile positive charge carrier as neighboring electrons shift to fill the vacancy.312

By controlling these charge carriers, engineers can tune the material's conductivity to create essential components like transistors and diodes.45