Comment by adrian_b

For high-voltage applications (like power supplies) or high-power high-frequency applications (like radio transmitters) or high temperature applications (e.g. inside ovens or engines) silicon transistors are already in the process of being completely replaced by silicon carbide or gallium nitride transistors.

Despite the fact that those are much more expensive than silicon transistors, they are so much better that their use saves money in other parts of the system in which they are used.

Diamond transistors could be even better than SiC and GaN transistors, but there are many difficult technological problems that must be solved until being able to make commercial products.

The linked page reports some significant advances, which avoid a source of problems in the past, which were caused by the poor behavior of dopant impurities inside diamond crystals.