  We predict the self-excitation of terahertz (THz) oscillations due to the
plasma instability in the lateral n+-i-n-n+$ graphene field-effect transistors
(G-FET). The instability is associated with the Coulomb drag of the
quasi-equilibrium electrons in the gated channel by the injected ballistic
electrons resulting in a positive feedback between the amplified dragged
electrons current and the injected current. The plasma excitations arise when
the drag effect is sufficiently strong. The drag efficiency and the plasma
frequency are determined by the quasi-equilibrium electrons Fermi energy (i.e.,
by their density). The conditions of the terahertz plasma oscillation
self-excitation can be realized in the G-FETs with realistic structural
parameters at room temperature enabling the potential G-FET-based radiation
sources for the THz applications.
