Semiconductor excitation by light. Animation showing the excitation of a semiconductor at low temperatures following a flash of light. The blue box represents the semiconductor, with the atoms colour-coded according to their energy levels: green (ground state), white (atoms with an electron lost to the conduction band). The light flash excites most of the electrons into the conduction band (free electrons, red) from where the electrons start to recombine until the animation ends. As the animation zooms in, the valence electrons orbiting the nuclei are visible in the atoms in their ground state. This is the basic principle behind an insulator or semiconductor. It is the free electrons in the conduction band that give rise to the electric conductivity of a solid. The excitation from the valence into the conduction band can be induced thermally (K004/3231) or (as here) by a flash of light.