Why is gallium arsenide (GaAs) useful for semiconductor devices operating at UHF and higher frequencies?
Why Electron mobility is how quickly charge carriers move through the crystal under a given electric field, and in gallium arsenide it is roughly five times that of silicon. Faster carriers mean shorter transit times through the device, so the transistor keeps useful gain up into the microwave region where silicon devices run out of steam. That same property also gives GaAs devices low noise, which is why GaAs FETs are favored for UHF and microwave preamps.
Watch out The choice mentioning higher noise figures is backwards: GaAs devices are prized precisely because their noise figures are very low, often well under 1 dB at UHF.
GaAs: electrons Go faster. High mobility equals high frequency and low noise.