Coupled mode effects in the stationary and transient behavior of squeezed channel field-effect transistors

While nanoscale multi-gate field-effect transistors (FETs) can mitigate unwanted short-channel effects, the quantum confinement of charge carriers significantly influences the device behavior, leading to issues, such as increased gain compression in RF amplifier applications. Simulating the quantum transport in these devices remains challenging, particularly in the transient case, so that the so-called mode-space approximation is often used. Even still, for devices where these modes couple with each other, no adequate time-resolved simulation methods exist. This is due to the fact that non-equilibrium Green’s function methods are virtually restricted to the steady-state analysis for these devices, while in transient density matrix approaches, the coupling between the modes is difficult to take into account and has been neglected in the past. We resolve this issue by applying the coupled mode-space approximation to a tight-binding Hamiltonian and inserting into a Heisenberg equation of motion for the density matrix. The resulting equation is solved in real space without applying a Fourier transform, eliminating the need for restrictive discretization patterns. On the contrary, the discretization pattern of our proposed method directly follows from the Hamiltonian that is used. We validate the approach against reference results obtained by real-space and mode-space non-equilibrium Green’s function methods. We demonstrate accurate modeling of mode coupling, even in devices with abruptly changing channel geometries. Finally, the effects of a channel constriction of gate-all-around FETs in amplifier operation are studied, where, even though the current densities differ, similar amplifier behavior for the devices with and without constriction is seen.