Why is silicon sensitive to visible light and some infrared light?

Silicon is sensitive to visible and some infrared light because of its specific electronic structure, which features a bandgap of approximately 1.12 electron volts.12

In a semiconductor like silicon, the valence band is filled with electrons, while the conduction band is empty. For an electron to move into the conduction band and contribute to electrical conductivity, it must receive enough energy to cross the bandgap.321

Photons of visible and infrared light carry energy that corresponds to this range. Specifically, because silicon's bandgap is 1.12 eV, any incident photon with energy equal to or greater than this threshold can excite an electron from the valence band to the conduction band, creating a hole-electron pair. This threshold energy corresponds to a wavelength of approximately 1,100 nanometres, which falls within the infrared spectrum. Consequently, silicon can effectively absorb and respond to both visible light and infrared radiation up to that wavelength.13