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.