Optical Versus Electronic Bandgap
In materials with a large exciton binding energy, it is possible for a photon to have just barely enough energy to create an exciton (bound electron-hole pair), but not enough energy to separate the electron and hole (which are electrically attracted to each other). In this situation, there is a distinction between "optical bandgap" and "electrical band gap" (or "transport gap"). The optical bandgap is the threshold for photons to be absorbed, while the transport gap is the threshold for creating a electron-hole pair that is not bound together. (The optical bandgap is at a lower energy than the transport gap.)
In almost all inorganic semiconductors, such as silicon, gallium arsenide, etc., there is very little interaction between electrons and holes (very small exciton binding energy), and therefore the optical and electronic bandgap are essentially identical, and the distinction between them is ignored. However, in some systems, including organic semiconductors, the distinction may be significant.
Read more about this topic: Band Gap
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