  The modulation depth of 2-D electron gas (2DEG) based THz modulators using
AlGaAs/GaAs heterostructures with metal gates is inherently limited to < 30%.
The metal gate not only attenuates the THz signal (> 90%) but also severely
degrades the modulation depth. The metal losses can be significantly reduced
with an alternative material with tunable conductivity. Graphene presents a
unique solution to this problem due to its symmetric band structure and
extraordinarily high mobility of holes that is comparable to electron mobility
in conventional semiconductors. The hole conductivity in graphene can be
electrostatically tuned in the graphene-2DEG parallel capacitor configuration,
thus more efficiently tuning the THz transmission. In this work, we show that
it is possible to achieve a modulation depth of > 90% while simultaneously
minimizing signal attenuation to < 5% by tuning the Fermi level at the Dirac
point in graphene.
