  Electrically injected terahertz (THz) radiation sources are extremely
appealing given their versatility and miniaturization potential, opening the
venue for integrated-circuit THz technology. In this work, we show that
coherent THz frequency combs in the range
$0.5~\mathrm{THz}<\omega/2\pi<10~\mathrm{THz}$ can be generated making use of
graphene plasmonics. Our setup consists of a graphene field-effect transistor
with asymmetric boundary conditions, with the radiation originating from a
plasmonic instability that can be controlled by direct current injection. We
put forward a combined analytical and numerical analysis of the graphene plasma
hydrodynamics, showing that the instability can be experimentally controlled by
the applied gate voltage and the injected current. Our calculations indicate
that the emitted THz comb exhibits appreciable temporal coherence
($g^{(1)}(\tau)>0.6$) and radiant emittance ($10^{7}\,\mathrm{Wm^{-2}}$). This
makes our scheme an appealing candidate for a graphene-base THz laser source.
Moreover, a mechanism for the instability amplification is advanced for the
case of substrates with varying electric permitivitty, which allows to overcome
eventual limitations associated with the experimental implementation.
