Why are boron and indium called acceptor atoms in semiconductors?

Boron and indium are classified as acceptor atoms because they are trivalent elements—meaning they possess only three electrons in their outer orbit—that are introduced into a semiconductor lattice, such as silicon.12

In a standard silicon crystal, each atom forms four covalent bonds with its neighbors. When a trivalent atom like boron or indium replaces a silicon atom, it has only three electrons available to participate in these bonds. To complete the four covalent bonds required by the surrounding lattice structure, the dopant atom must "accept" an electron from the crystal.123

This process leaves behind an electron vacancy in the valence band, which is referred to as a hole. Because this hole acts as a positively charged carrier that can migrate through the lattice, the resulting material is known as a p-type semiconductor, and the dopant atom is designated as an acceptor.123