Plasma instability and amplified mode switching effect in THz field effect transistors with a grating gate

We developed a theory of collective plasma oscillations in a dc current-biased field effect transistor with an 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 a small instability increment experiencing redshift with increasing current changes to the blue-shifting peak with a large instability increment. This amplified mode switching effect has been recently observed in graphene-interdigitated structures. 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 electromagnetic radiation.