How does thermal energy create electron-hole pairs in a pure semiconductor?

In a pure (intrinsic) semiconductor, electrons are typically bound within the crystalline lattice in covalent bonds, occupying the valence band while the conduction band remains empty.123

At higher temperatures, thermal vibrations provide sufficient energy to break some of these covalent bonds. When an electron gains enough thermal energy to overcome the bandgap—the energy range that electrons cannot normally occupy—it is promoted from the valence band to the conduction band, becoming a free conduction electron.132

This process leaves behind an electron vacancy in the valence band known as a hole. Because the electron has moved to a higher energy state, this resulting pair of a free electron and a hole is referred to as an electron-hole pair. Both carriers can then contribute to electrical conduction when an electric field is applied.123