Topic summary
Bandgap
Extracted from the Wikipedia article Band gap.
Optical versus electronic bandgap
It is possible for an absorbed photon to have just enough energy to generate a bound electron–hole pair (an exciton), but not enough additional energy to separate the two against their mutual electrical attraction. The former energy is called the "optical band gap", while the energy to create an unbound pair so that a macroscopic electrical current will flow is the "electronic band gap" or "transport gap", and is greater than the former by the exciton binding energy.