for semiconductors and insulators, the energies that lie between the valence and conduction bands.
for semiconductors and insulators, the energies that lie between the valence and conduction bands.
For semiconductors and insulators, the energies that lie between the valence and conduction bands. 
The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.