Semiconductor devices have been fabricated from a large number of alloys of the groups 3 and 5 element, but the heaviest element in these groups, thallium and bismuth, have largely been neglected.
These element are difficult to incorporate into the lattice due to their large size and tendency to surface segregate.
This tendency to surface segregate has made bismuth a useful surfactant during the growth of other 3 / 5 semiconductor materials.
Incorporation of small amounts of bismuth produces a large reduction in the bandgap of GaAs.