Gallium arsenide ($\mathrm{GaAs}$)

Gallium arsenide is the binary semiconductor $\mathrm{GaAs}$, containing one gallium atom per arsenic atom in its ideal composition. Its common zinc-blende-related structure is distinct from a simple mixture of elemental gallium and arsenic.

Its direct band gap supports optical transitions useful in emitters and photodetectors. Crystal defects, doping, and interfaces affect device behaviour in addition to the bulk chemical formula.

A zone-centre effective-mass description retaining the s-like conduction band, heavy-hole and light-hole branches of the p-like $j=3/2$ valence manifold, and the spin-orbit-split $j=1/2$ valence branch. Each named branch includes spin multiplicity; four bands does not mean four individual quantum states.