How can light change the electrical conductivity of a semiconductor?

Light increases the electrical conductivity of a semiconductor through a process known as photoconductivity.1

In a semiconductor, most electrons are bound within the valence band, leaving the conduction band nearly empty and the material a relatively poor conductor. However, when the material is exposed to light with photons of sufficient energy, these photons can transfer their energy to electrons. If the photon energy exceeds the band gap—the forbidden energy region between the valence and conduction bands—it excites electrons from the valence band into the conduction band.213

Once in the conduction band, these electrons are free to move and carry an electric current. This excitation also creates "holes" (missing negative charges) in the valence band, which act as positive charge carriers. Both the freed electrons and the holes contribute to an increase in the material's electrical conductivity. This phenomenon is widely utilized in light-sensitive devices, such as photovoltaic cells and various light-controlled switches.132