An intrinsic semiconductor, such as pure silicon, has electrical conductivity between a conductor and an insulator. In this pure crystal, atoms are held by covalent bonds, locking valence electrons in place and resulting in low conductivity. To transform this material for electronics, doping is used, which involves deliberately introducing trace amounts of impurity atoms. This introduction radically changes the material’s electrical behavior by creating a surplus of mobile charge carriers.
The Role of Phosphorus in Silicon
Phosphorus is used to create an N-type semiconductor. As a Group 15 element, it is a pentavalent atom possessing five valence electrons. When a phosphorus atom is added into the silicon crystal lattice, it substitutes a silicon atom that has four valence electrons. Since phosphorus contributes an extra electron beyond what is needed for bonding, it is classified as a donor impurity. The resulting material is designated as N-type (‘N’ for negative), indicating that the primary charge carriers are electrons.
How N-Type Doping Creates Free Electrons
The mechanism for creating a free electron is rooted in the phosphorus atom’s structure within the silicon lattice: four of the five valence electrons form stable covalent bonds with surrounding silicon neighbors, while the fifth is not needed for bonding and remains loosely bound to the nucleus. This extra electron occupies the donor level, which lies just below the semiconductor’s conduction band. Because the energy difference is extremely small, only minimal thermal energy is required to set the electron free. Once mobilized, this electron moves freely throughout the crystal lattice, ready to conduct electrical current and becoming the majority carrier. The phosphorus atom, having lost an electron, becomes a fixed, positively charged ion that does not contribute to current flow.
Contrast with P-Type Doping
The N-type doping mechanism contrasts sharply with the process used to create P-type semiconductors. P-type materials are created by doping silicon with Group 13 elements, such as boron or aluminum, which are trivalent and possess only three valence electrons. When a trivalent atom replaces a silicon atom, it forms only three covalent bonds, leaving a deficit of one electron, referred to as a “hole.” P-type dopants are known as acceptor impurities because they readily accept an electron from a neighboring silicon atom to complete their four bonds. This acceptance causes the hole to effectively move, allowing it to act as a mobile positive charge carrier. These holes become the majority carriers in the material, giving it the positive, or ‘P,’ designation.