  The plasma wave instabilities in ballistic Field Effect Transistors (FETs)
have a promise of developing sensitive THz detectors and efficient THz sources.
One of the difficulties in achieving efficient resonant plasmonic detection and
generation is assuring proper boundary conditions at the contacts and at the
heterointerfaces and tuning the plasma velocity. We propose using the tunable
narrow channel regions of an increased width, which we call "stubs" for
optimizing the boundary conditions and for controlling the plasma velocity. We
developed a compact model for THz plasmonic devices using the transmission line
(TL) analogy. The mathematics of the problem is similar to the mathematics of a
TL with a stub. We applied this model to demonstrate that the stubs could
effectively control the boundary conditions and/or the conditions at
interfaces. We derived and solved the dispersion equation for the device with
the stubs and showed that periodic or aperiodic systems of stubs allow for
slowing down the plasma waves in a controllable manner in a wide range. Our
results show that the stub designs provide a way to achieve the optimum
boundary conditions and could also be used for multi finger structures - stub
plasmonic crystals - yielding better performance of THz electronic detectors,
modulators, mixers, frequency multipliers and sources.
