Direct and indirect bandgap refers to two ways a semiconductor’s Bandgap can be arranged, which decides whether the material 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 cannot 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; instead 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 in this course — diodes, MOSFETs, BJTs — precisely 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 Light-emitting diode’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.)
[Background from general knowledge, not the source PDF]