[madengr]

GaN has a higher breakdown voltage than Si, and a higher drift velocity, which allows a faster switching speed (for the equivalent bias).

For power supplies and switched mode RF amplifiers, you need to switch the transistor on and off as fast as possible; ideally an open or short circuit. During the time is is transitioning from on to off, it is dissipating power, thus losing efficiency in your circuit.

If you can operate at a higher bias, your passive support circuitry can operate at a lower current (lower I^2*R losses) not to mention smaller due to higher frequency (smaller inductors and capacitors).

The faster switching also generates more harmonics, which can align in phase to cause higher voltage transients, thus you need a higher breakdown.

The issue with GaN is fabricating it. It is a compound semiconductor grown with epitaxy (layer by layer) and has to be lattice matched to a carrier substrate, unlike Si which is diffusion doped into the bulk Si substrate. So GaN is maybe at 8” wafers while Si is much bigger. Some GaN is on Si carrier (cheaper but lower thermal conductivity) and some in SiC (expensive but better thermal conductivity). The goal is to grow it on diamond, but again the lattice matching is a problem.

The longer term goal is diamond semiconductors. Maybe 30 years from now that will replace GaN, as GaN is doing to GaAs and Si for certain applications.