[jpmattia]

> * I wonder if they have considered them as power switching devices.*

It's been a couple of decades since I last looked: The diamond thin-film guys thought that power would be a sensible application. There is a technological hurdle: If you notice in the diagram, the electrodes are pointed. This increases the field at the tip, in order to overcome the electric-potential barrier to emission of the electrons. Reliability issues arise because the emission is occurring over a smaller surface area.

Like I said, it's been a while since I last looked, but vacuum devices have had appeal for power apps for a long time.