Aluminium gallium arsenide (also aluminum gallium arsenide) (AlxGa1-xAs) is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger bandgap. The x in the formula above is a number between 0 and 1 - this indicates an arbitrary alloy between GaAs and AlAs.
The bandgap varies between 1.42 eV (GaAs) and 2.16 eV (AlAs). For x < 0.4, the bandgap is direct.
The formula AlGaAs should be considered an abbreviated form of the above, rather than any particular ratio.
Aluminium gallium arsenide is used as a barrier material in GaAs based heterostructure devices. The AlGaAs layer confines the electrons to a gallium arsenide region. An example of such a device is a quantum well infrared photodetector (QWIP).
It can also be used in 1064 nm (Infra-red) laser diodes.
Read more about Aluminium Gallium Arsenide: Safety and Toxicity Aspects