Acceptors are elements added in the doping process that have one less electron in the valence shell than the substrate semiconductor material. With one less electron, this dopant “accepts” or pulls away an electron from the base semiconductor. By pulling away this extra electron, it creates a new “hole” in the semiconductor lattice. This creates a p-type semiconductor material.
Acceptors are trivalent impurity atoms that can take an electron. They are added during the doping process to create a p-type semiconductor.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
The diffused impurities with three valence electrons are called acceptor atoms.
Electronic Devices and Circuit Theory, 11th Edition by Robert L. Boylestad & Louis Nashelsky
For example, when silicon (Si), having four valence electrons, needs to be doped as a p-type semiconductor, elements from group III like boron (B) or aluminium (Al), having three valence electrons, can be used. The latter elements are also called trivalent impurities. Other trivalent dopants include indium (In) and gallium (Ga).
When substituting for a Si atom in the crystal lattice, the three valence electrons of boron form covalent bonds with three of the Si neighbours but the bond with the fourth neighbour remains unsatisfied. The initially electro-neutral acceptor becomes negatively charged (ionised). The unsatisfied bond attracts electrons from the neighbouring bonds. At room temperature, an electron from a neighbouring bond will jump to repair the unsatisfied bond thus leaving a hole (a place where an electron is deficient). The hole will again attract an electron from the neighbouring bond to repair this unsatisfied bond. This chain-like process results in the hole moving around the crystal able to carry a current thus acting as a charge carrier.