Which of the following materials supports the highest frequency of operation when used in MMICs?
Why Monolithic microwave integrated circuits need a substrate whose charge carriers move very fast, and compound III-V semiconductors like gallium nitride and gallium arsenide have much higher electron mobility and saturation velocity than silicon. Gallium nitride also has a wide bandgap, so it handles high voltage and power density at microwave frequencies. That combination is why GaN devices dominate modern high-frequency, high-power RF amplifier stages.
Watch out Silicon is a real semiconductor but its lower carrier mobility limits it to lower frequencies, and silicon nitride and silicon dioxide are not semiconductors at all, they are insulating or dielectric layers used for passivation and capacitors.
Three choices start with 'silicon'; the odd one out, gallium nitride, is the microwave winner.