“It was the idea of the bandgap that allowed people to understand and harness semiconductors for optoelectronic devices” — that is, devices that work with light and electricity — says Tonio Buonassisi, MIT’s SMA Assistant Professor of Mechanical Engineering and Manufacturing.
When electrons get excited (by getting heated, or by being hit with a particle of light, known as a photon), they can jump across the gap. If an electron in a crystal gets hit by a photon that has enough energy, it can get excited enough to jump from the valence band to the conduction band, where it is free to form part of an electric current. That’s what happens when light strikes a solar cell, producing a flow of electrons.
Silicon, a semiconductor, is the material of choice for solar cells in large part because of its bandgap. Silicon’s bandgap is just wide enough so that electrons can easily cross it once they are hit by photons of visible light.
The same process also works in reverse. When electricity passes through a semiconductor, it can emit a photon, whose color is determined by the material’s bandgap. That’s the basis for light-emitting diodes, which are increasingly being used for displays and computer screens, and are seen as the ultimate low-power light bulbs
PS SILICON FOUND IN SAND IN ABUNDANCE IS IDEAL MATERIAL WITH THE DESIRED PROPERTIES. THOUG PROCESS FOR MAKING THE SILICON WAFERS IS EXPENSIVE,DIFFICULT AND ENERGY INTENSIVE.