Why Gallium arsenide has much higher electron mobility than silicon, so carriers cross the device faster and it can amplify or switch at far higher frequencies with less noise. That makes GaAs the material of choice for microwave devices such as GaAs FETs, low-noise preamplifiers and monolithic microwave integrated circuits. Its semi-insulating substrate also cuts stray capacitance and loss at GHz frequencies.
Watch out High-current rectifiers and high-power audio stages are silicon (or silicon carbide) territory, where GaAs offers no advantage and costs far more; low-frequency RF is easily handled by ordinary silicon devices.
GaAs = Gigahertz Amplifier stuff. Think microwave preamp, not power rectifier.