III-V semiconductors

III-V semiconductors are the materials photonics emits with. Combining group-III elements (Ga, In, Al) with group-V (As, P, N, Sb) produces crystals whose band gaps are direct in most technologically important compositions: an electron and hole recombine without phonon assistance, so radiative recombination is fast and efficient. Silicon's indirect gap forbids exactly this, which is the one-sentence explanation of why lasers are III-V and logic is silicon.

The engineering power is alloying: mixing on each sublattice tunes band gap and lattice constant semi-independently. The families sort by substrate:

GaAs-based (AlGaAs, InGaAs QWs): 780–1100 nm. VCSELs for datacom and sensing, high-power pump bars, consumer laser diodes. InP-based (InGaAsP, InGaAlAs): 1200–1700 nm, the telecom windows. DFBs, EAMs, SOAs, high-speed PIN and APD detectors, and full monolithic transmitter/receiver PICs. GaN-based: UV through green emitters. Solid-state lighting and blue lasers. GaSb-based and quantum-cascade structures on InP/GaAs: the mid-IR, for sensing and spectroscopy.

Heterostructure freedom is the second superpower. Because ternaries and quaternaries lattice-match their substrates at tunable gaps, III-Vs stack abrupt band-offset layers at will: double heterostructures, quantum wells 5–10 nm thick, strained layers that reshape the valence band to cut threshold current. Every semiconductor laser entry in this glossary rests on that capability.

The limitations are industrial: wafers are small (2–6", versus silicon's 300 mm), fragile, and expensive; processing ecosystems are subscale beside CMOS. Modern photonics therefore treats III-V as the active ingredient, grown epitaxially on native substrates, then attached to silicon or nitride circuits by flip-chip bonding, die/wafer bonding (heterogeneous integration), or, prospectively, direct hetero-epitaxy of quantum-dot lasers on silicon.