  We developed a theory of collective plasma oscillations in a dc
current-biased field effect transistor with interdigitated dual grating gate
and demonstrated a new mechanism of electron plasma instability in this
structure. The instability in the plasmonic crystal formed in the transistor
channel develops due to conversion of the kinetic energy carried by the
drifting plasmons into electromagnetic energy. The conversion happens at the
opposite sides of the gate fingers due to the asymmetry produced by the current
flow and occurs through the gate finger fringing capacitances. The key feature
of the proposed instability mechanism is the behavior of the plasma frequency
peak and its width as functions of the dc current bias. At a certain critical
value of the current, the plasma resonant peak with small instability increment
experiencing redshift with increasing current changes to the blue shifting peak
with large instability increment. This amplified mode switching (AMS) effect
has been recently observed in graphene-interdigitated structures (S.
Boubanga-Tombet et al., Phys. Rev. X 10, 031004 (2020)). The obtained
theoretical results are in very good qualitative agreement with these
experiments and can be used in future designs of the compact sources of THz EM
radiation.
