  The terahertz detection performance and operating regimes of graphene
plasmonic field-effect transistors (FETs) were investigated by a hydrodynamic
model. Continuous wave detection simulations showed that the graphene response
sensitivity is similar to that of other materials including Si, InGaAs, GaN,
and diamond-based FETs. However, the pulse detection results indicated a very
short response time, which favors the rapid/high-sensitively detection. The
analysis on the mobility dependence of the response time revealed the same
detection regimes as the traditional semiconductor materials, i.e. the
non-resonant (collision dominated) regime, the resonant ballistic regime, and
the viscous regime. When the kinematic viscosity ({\nu}) is above a certain
critical viscosity value, {\nu}NR, the plasmonic FETs always operates in the
viscous non-resonant regime regardless of channel length (L). In this regime,
the response time rises monotonically with the increase of L. When {\nu} <
{\nu}NR, the plasmonic resonance can be reached in a certain range of L (i.e.
the resonant window). Within this window, the carrier transport is ballistic.
For a sufficiently short channel, the graphene devices would always operate in
the non-resonant regime regardless of the field-effect mobility, corresponding
to another viscous regime. The above work mapped the operating regimes of
graphene plasmonic FETs, and demonstrated the significance of the viscous
effects for the graphene plasmonic detection. These results could be used for
the extraction of the temperature dependences of viscosity in graphene.
