What is the highest frequency possible to be generated with electronic circuits
http://spectrum.ieee.org/semiconductors/devices/transceivers-to-conquer-the-terahertz-frontier/2 is a good read to start from.

There isn't a hard upper limit, but it gets increasingly painful to work with and starts crossing over into the domain of optics and optoelectronics. One of the limits mentioned there is that it takes a distinct amount of time for a signal to travel through a transistor, so transistors become unusable; that article claims this is somewhere in the hundreds of GHz.

Parasitic capacitance also becomes an issue; the higher the frequency, the easier it is for the signal to just leak out through the few pF capacitance between your wire and the nearest other bit of metal. This effect limits the usable frequency of signals on breadboards to a few MHz.