The band gap energy (E_g) is the minimum energy required to break a covalent bond and thus, generates : 

Explanation:

• The band gap energy (Eg) is the minimum energy required to break a covalent bond and thus, generates an electron-hole pair
• The energy gap is the shortest distance between the valence layer and the conduction layer
• The Energy required for a photon to create an e-h pair is a little bit larger than the band gap in the case of an indirect band-gap semiconductor