Why is gallium arsenide (GaAs) useful for semiconductor devices operating at UHF and higher frequencies?

Explication

Electrons drift through gallium arsenide several times faster than through silicon for the same electric field, so carriers cross the device in less time and useful gain extends into the UHF and microwave range. GaAs devices also tend toward lower noise figures rather than higher, and it is mobility rather than junction drop or transconductance that buys the frequency.