Aluminium gallium arsenide
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Aluminium gallium arsenide (also Aluminum gallium arsenide) (Al_xGa_1-xAs) is a semiconductor with the 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).

The formula AlGaAs should be considered an abbreviated form of the above, rather than any particular ratio.

The original article can be found at: http://en.wikipedia.org/wiki/Aluminium_gallium_arsenide.
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