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-FETs). 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 electron 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 THz applications.
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