Subharmonic Mixing With Field-Effect Transistors: Theory and Experiment at 639 GHz High Above $f_{T}$

We present experimental and theoretical results on subharmonic mixing in field-effect transistors high above the transistor cutoff frequencies $f_{T}$ . Analytical expressions for heterodyne and subharmonic mixing are derived considering different coupling conditions. They have to ensure that the charge density oscillations excited in the transistors\´ channels by the mixing signals overlap spatially. If this is the case, then a high efficiency of the mixing process can be sustained even at frequencies much above cutoff; in fact, the efficiency is predicted to increase with rising frequency when the plasma-wave regime $\omega \tau \geq 1$ , $\tau$ being the electron momentum scattering time, is reached. With patch-antenna-coupled zero-drain-bias (passive) mixers, which we have implemented in standard a 150-nm silicon complementary metal–oxide–semiconductor process technology, subharmonic mixing is demonstrated for a signal frequency of 639 GHz and a local-oscillator frequency of 213 GHz (fundamental antenna resonance and its third harmonic). A local-oscillator drive of 8.2 mV yields an amplitude conversion of the electrical voltage of ${-}{\rm 56}~{\rm dB}$ . The experimentally determined conversion fully agrees with the derived theory.