ircuits that (accidentally) oscillated around 12 GHz, using 40-GHz SiGe bip olar transistors. You can get discretes with f_T values of 80 GHz or more (BFP840 iirc).
riend of mine from grad school, Mark Rodwell, has been doing great things i n that line for some years now out at UCSD.
travelling-wave tubes (TWTs), all of which still have their uses. For power s of milliwatts to watts, there are negative-resistance diodes such as Gunn and IMPATT (impact-avalance transit time), and for low power, high-order f requency multipliers such as step-recovery diodes running off lower frequen cy oscillators.
sentially any frequency you like by picking two teeth of the comb and beati ng them together, e.g. with a nonlinear crystal or a fast photodetector suc h as an antenna-coupled tunnel junction.
A Happy New Year to each of you who responded to my query, and thanks for the helpful insights. Over the past two years, I have been involved deeply in writing custom software for analysis/ design of distributed filters and related RF/microwave devices, and naturally one thing led to another, and I am getting i nvolved RF/ultra high frequency amplifier and oscillator analysis.