band gap
/ band gap /
Picture the allowed energies of a crystal as floors in a building where electrons can live. The energy bands are the floors — broad ranges packed with rooms. But between some floors there is no floor at all: a stretch of energy where an electron simply cannot exist, no matter how hard it tries. That empty, forbidden stretch is the band gap.
A band gap is a range of energies in which there are no allowed electron states. An electron in the band below the gap cannot have an energy somewhere inside the gap; to cross it, the electron must absorb a chunk of energy at least as big as the gap and jump clean to the band above. The size of the gap, usually measured in electron-volts, is one of the single most important numbers describing a material.
Band gaps matter because they govern whether electricity flows and which colours of light a material absorbs or emits. A roughly one-electron-volt gap gives a useful semiconductor; a several-electron-volt gap gives an insulator; no gap at all gives a metal. A frequent confusion is to think the gap is a physical space between atoms — it is not; it is a gap in energy, an empty band on the ladder of allowed energies.
Silicon has a band gap of about 1.1 electron-volts, while diamond — chemically a cousin of silicon — has a gap of about 5.5 electron-volts. That difference is why a silicon chip can be coaxed into conducting and switching, while clear diamond stays a stubborn insulator that lets visible light pass straight through.
A small gap (silicon) makes a semiconductor; a large gap (diamond) makes an insulator.
Band gap and energy gap mean the same thing in everyday use; this domain lists energy gap separately to stress that it is the measured size of the forbidden region, while band gap names the forbidden region itself.