What is the process called when electrons fill vacancies in the valence band?

The process where electrons fill vacancies in the valence band is known as recombination. This phenomenon typically occurs when free electrons, which may have been excited to a conduction band by energy absorption, lose energy and transition back to the valence band. During this transition, they fill the vacancies (or holes) created in the valence band, effectively neutralizing those missing electrons.

Recombination is fundamental in understanding the behavior of semiconductors and optical devices, as it plays a critical role in the generation and annihilation of charge carriers, influencing the conductivity and light emission properties of materials.

The other options represent distinct processes. Condensation refers to the phase transition from gas to liquid and is unrelated to charge carriers in a solid. Excitation involves the process of raising electrons from lower energy states to higher energy states, which is not filling vacancies. Doping involves introducing impurities into a semiconductor to modify its electrical properties but does not directly describe the filling of vacancies in the valence band.