I work in millimeter- and submillimeter-wave radio astronomy, where we receive signals in the 60 to 800 GHz range. There is a lot of water absorption of the signal, leaving only certain frequency bands usable. The semiconductors used at these frequencies are rather exotic - we use superconducting materials in our receivers for lower noise, and we cool them to 4 Kelvin. Making a power amplifier to produce even one Watt of signal at 75 GHz is a million-dollar project. In short, it's not likely to be mainstream for at least ten years.