How a semiconductor’s Bandgap is arranged decides whether it can emit light efficiently. A direct-gap material releases the recombination energy as a photon; an indirect-gap material wastes it as heat. This is the single reason a Light-emitting diode is made from gallium compounds and not from silicon.
What “direct” vs “indirect” means
Electrons in a crystal have both an energy and a momentum. Plotting electron energy against momentum, the conduction band has a lowest point (the bottom of the band) and the valence band has a highest point (the top). In a direct-bandgap material (GaAs, GaN, and their ternary alloys) those two extrema sit at the same momentum. In an indirect-bandgap material (silicon, germanium) the conduction-band minimum is at a different momentum from the valence-band maximum.
Why this controls light emission
When an electron recombines with a hole it must conserve both energy and momentum. A photon carries plenty of energy () but almost no momentum. So:
- Direct gap: the electron drops straight down at the same momentum. Energy is conserved by emitting a photon; momentum is already conserved because there is no momentum change. Recombination → photon, efficiently. Good for LEDs.
- Indirect gap: the electron must also change momentum to get from the conduction-band minimum to the valence-band maximum. A photon can’t supply that momentum, so the transition needs a lattice vibration (a phonon) to participate as well. A three-body event (electron, photon, phonon) is far less likely, so the energy overwhelmingly comes out as heat (phonons). Recombination → mostly heat. Useless for LEDs.
The practical consequence
Silicon is the workhorse of every other circuit here (diodes, MOSFETs, BJTs) because its indirect gap makes it cheap, stable, and easy to process. But that same indirect gap makes it a terrible light emitter, so LEDs and laser diodes are built from direct-gap III–V compounds instead. Choosing the compound also chooses the bandgap and therefore the LED’s colour. Light detection is less fussy: a Photodiode can be silicon, because absorbing a photon with phonon help is far more tolerable than emitting one.