That's due to thermal and electrical limits of Silicon and whatever you are using to insulate (Silicon Dioxide)... having a semiconductor with a higher bandgap (Such as many III-V semiconductors, such as indium gallium arsenide), you can have a much higher clockspeed.
I'm really interested in this, could you please explain more? My impression is that the heat from subthreshold current is what's limiting today's CPUs. How does a higher bandgap help you achieve a higher clockspeed for the same power density?
