Limitations in THz power generation with Schottky diode varactor frequency multipliers

We discuss the limitations in power generation with Schottky diode and HBV (heterostructure barrier varactor) diode frequency multipliers. It is shown that at lower frequencies the experimental results achieved so far approach the theoretical limit of operation for the employed devices. However, at increasing frequencies the power drops with f^-3 instead of the f^-2 predicted by theory. In this contribution we provide an overview of state-of-the-art results. A comparison with theoretically achievable multiplier performance reveals that the devices employed at higher frequencies are operating inefficiently and the design and fabrication capabilities have not reached the maturity encountered at lower THz frequencies.