https://phys.org/news/2018-05-silicon-breakthrough-key-microwave-technology.html

High-frequency microwaves carry signals in a wide range of devices, including the radar units police use to catch speeders and collision-avoidance systems in cars.

The microwaves are typically generated by devices called Gunn diodes, which take advantage of the unique properties of expensive and toxic semiconductor materials such as gallium arsenide.