energy gap
/ EN-ur-jee gap /
Think of climbing a staircase where, partway up, several steps are simply missing, leaving a sheer wall you must clear in one jump. You cannot stop halfway up that wall — there is nothing to stand on. The height of that unclimbable stretch is what physicists call the energy gap: the size of the forbidden region an electron must leap clean across.
An energy gap is the size, measured in energy, of a forbidden range separating two electronic bands. If the band gap is the empty region itself, the energy gap is the number that tells you how wide it is — usually quoted in electron-volts. An electron sitting in the band below cannot gain just any energy; to reach the band above it must absorb at least the full gap's worth in one go, whether from heat, light, or an electric field.
The energy gap matters because its size, often just a single number, sets a material's electrical and optical character: roughly how easily it conducts, and which colours of light it can absorb or emit. The common confusion is treating energy gap and band gap as different things — in ordinary usage they are the same forbidden region, with energy gap simply emphasising its measured magnitude. The same idea of a forbidden energy span also appears far beyond bands, for instance in the gap that opens up in a superconductor.
A red LED is built from a semiconductor whose energy gap is about 1.9 electron-volts. When an electron drops across that gap, it releases exactly that much energy as a single particle of light — and a packet of about 1.9 electron-volts is precisely the energy of red light. The gap size literally chooses the colour.
The size of the energy gap fixes the colour of light an LED emits.
In some materials the lowest point of the upper band sits directly above the highest point of the lower band, while in others it is offset sideways in momentum; this direct-versus-indirect distinction strongly affects how efficiently the material can emit light, even at the same gap size.