In physics of semiconductors, an acceptor is a dopant atom (impurity) that, when added to a semiconductor, can form a p-type semiconductor. The process of adding controlled impurities to a semiconductor is known as semiconductor doping. This process changes an intrinsic semiconductor to an extrinsic semiconductor. For both types of donor or acceptor atoms, increasing dopant density increases conductivity.
An extrinsic semiconductor which has been doped with electron acceptor atoms is called a p-type semiconductor, because the majority of charge carriers in the crystal are electron holes (positive charge carriers). The pure semiconductor silicon is a tetravalent element, the normal crystal structure contains 4 covalent bonds from four valence electrons. In silicon, the most common dopants are group III and group V elements. Group III elements (trivalent) all contain three valence electrons, causing them to function as acceptors when used to dope silicon. When an acceptor atom replaces a tetravalent silicon atom in the crystal, a vacant state (an electron hole) is created. An electron hole (often simply called a hole) is the lack of an electron at a position where one could exist in an atom or atomic lattice. It is one of the two types of charge carriers that are responsible for creating electric current in semiconducting materials. These positively charged holes can move from atom to atom in semiconducting materials as electrons leave their positions. The addition of trivalent impurities such as boron, aluminum or gallium to an intrinsic semiconductor creates these positive electron holes in the structure. For example, a silicon crystal doped with boron (group III) creates a p-type semiconductor whereas a crystal doped with phosphorus (group V) results in an n-type semiconductor.
The number of electron holes are completely dominated by the number of acceptor sites. Therefore:
The total number of holes is approximately equal to the number of donor sites, p ≈ NA.
Charge neutrality of this semiconductor material is also maintained. The net result is that the number of electron holes is increased, while the number of conduction electrons is reduced. The imbalance of the carrier concentration in the respective bands is expressed by the different absolute number of electrons and holes. Electron holes are majority carriers, while electrons are minority carriers in p-type material.
From the energy gap viewpoint, such impurities “create” energy levels within the band gap close to the valence band so that electrons can be easily excited from the valence band into these levels, leaving mobile holes in the valence band. They create “shallow” levels, levels that are very close to the valence band, so the energy required to ionize the atom (accept the electron that fills the hole and creates another hole further from the substituted atom) is small. This shifts the effective Fermi level to a point about halfway between the acceptor levels and the valence band. Fermi level is the term used to describe the top of the collection of electron energy levels at absolute zero temperature. The Fermi level is the surface of Fermi sea at absolute zero where no electrons will have enough energy to rise above the surface. In pure semiconductors the position of the Fermi level is within the band gap, approximately in the middle of the band gap.