Crystals can be semiconductors because the energy interval between the electron-filled valence band and the electron-filled conduction band in ultra-pure silicon is smaller than that of the insulator, generally 0.5-0.3eV.
The valence band is a low energy band formed by the energy levels of electron filled atomic orbitals, and any energy changes usually occur in the valence band. When illuminated or heated, valence band electrons are easily excited to the conduction band, and valence band electrons are excited to the conduction band, leaving a \"hole\