Feedback enhanced Dyakonov–Shur instability in graphene field-effect transistors

Abstract

Graphene devices are known to have the potential to operate THz signals. In particular,
graphene field-effect transistors have been proposed as 
devices to host plasmonic instabilities in the THz realm; for instance, Dyakonov-Shur instability which relies upon
dc excitation. 
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In this work, starting from a hydrodynamical description of the charge carriers, we extend the transmission line description of graphene field-effect transistors to a scheme with a positive feedback loop, also considering the effects of delay, which leads to the transcendental transfer function with terms of the form $e^{a s}\sech^k(s)/s$.
Applying the conditions for the excitation of Dyakonov--Shur instability, we report an enhanced voltage gain in the linear regime that is corroborated by our simulations of the nonlinear hydrodynamic model for the charge carriers. This translates to both greater saturation amplitude -- often up to 50\% increase -- and faster growth rate of the self-oscillations. Thus, we bring forth a prospective concept for the realization of a THz oscillator suitable for future plasmonic circuitry.