Acceptor Atom
An acceptor atom is an impurity atom with one fewer valence electron than the host atom it replaces, such as boron, aluminum, gallium or indium in silicon, which captures an electron from a neighboring bond and thereby creates a mobile hole. After accepting that electron, the acceptor is a fixed negative ion locked into the crystal.
An acceptor works by leaving one bond incomplete and letting the crystal's own electrons fill it. Boron has three valence electrons, so on a silicon lattice site it can complete only three of the four bonds with its neighbors. An electron from a nearby silicon-silicon bond can move to the boron atom for a small energy cost, which turns the boron into a negative ion and leaves a hole in the valence band, free to move. The energy needed is the acceptor ionization energy, about 0.045 electron-volts for boron in silicon, with the level sitting that far above the top of the valence band. This is again a hydrogen-like picture, but with a hole bound to a negative core, scaled down by the dielectric constant of about 11.7 and the hole effective mass.
Acceptors are usually fully ionized at room temperature, which is why boron is the standard p-type dopant. Thermal energy near 0.026 electron-volts is comparable to the 0.045 electron-volt ionization energy, and the valence band offers a large supply of electrons to give, so nearly every boron atom has accepted an electron at ordinary doping levels. Boron is also small enough to fit easily on silicon lattice sites, and it has high solubility, so concentrations from 10¹⁵ to above 10²⁰ atoms per cubic centimeter are achievable. Indium, whose level lies much deeper at about 0.16 electron-volts, ionizes only partly at room temperature, which restricts it to special uses such as certain retrograde channel-doping profiles and infrared detectors. Gallium and aluminum are intermediate and appear in diffused junctions and in solar cell and power-device processes.
The negative ion left behind is as important as the hole, because fixed acceptor charge shapes every junction and transistor. On the p side of a pn junction, the holes that diffuse across to the n side leave uncompensated negative boron ions behind, and the resulting field is half of what builds the depletion region and the built-in potential. Under the gate of an n-channel MOS transistor the gate voltage drives holes away and exposes this negative charge before it attracts electrons to form a channel, so the amount of boron in the channel region directly sets the threshold voltage. A common mistake is to picture acceptors as atoms that carry positive charge because they create positive holes; the neutral boron atom is not positive, and the crystal is neutral until, for example, a junction separates holes from their parent ions.
Boron's chemistry has everyday consequences in manufacturing and in solar power. Boron atoms diffuse faster than arsenic and can be driven in by heat or placed by ion implantation, with their tail controlled by short anneals, which is why the junction depth of the p-type source and drain of a PMOS transistor is a persistent design challenge. In the standard boron-doped, oxygen-containing solar-grade silicon, a boron-oxygen complex forms under illumination and gradually reduces cell efficiency, a loss that gallium-doped wafers largely avoid. Acceptors also exist outside silicon: gallium-arsenide uses zinc or carbon, and gallium nitride uses magnesium, whose deep acceptor level makes p-type nitride harder to make and was the central obstacle overcome in the development of blue light-emitting diodes. Compensation again applies, since donors cancel acceptors one for one and only the net acceptor density sets the hole count.
Read acceptor atom as the dopant that borrows one electron from the crystal, leaving behind a mobile hole and a fixed negative ion that keep the material neutral.