  Although time-independent models provide very useful dynamical information
with a reduced computational burden, going beyond the quasi-static
approximation provides enriched information when dealing with TeraHertz (THz)
frequencies. In this work, the THz noise of dual-gate graphene transistors with
DC polarization is analyzed from a careful simulation of the time-dependent
particle and displacement currents. From such currents, the power spectral
density (PSD) of the total current fluctuations are computed at the source,
drain and gate contacts. The role of the lateral dimensions of the transistors,
the Klein tunneling and the positive-negative energy injection on the PSD are
analyzed carefully. Through the comparison of the PSD with and without
Band-to-Band tunneling and graphene injection, it is shown that the unavoidable
Klein tunneling and positive-negative energy injection in graphene structures
imply an increment of noise without similar increment on the current, degrading
the (either low or high frequency) signal-to-noise ratio. Finally, it is shown
that the shorter the vertical height (in comparison with the length of the
active region in the transport direction), the larger the maximum frequency of
the PSD. As a byproduct of this result, an alternative strategy (without length
scaling) to optimize the intrinsic cut-off frequency of graphene transistors is
envisioned.
