Quantum transport in two- and three-dimensional nanoscale transistors: Coupled mode effects in the nonequilibrium Green’s function formalism

In this article, we study the coupled mode space approach to nonequilibrium Green’s function (NEGF) simulation. When the lateral confinement of nanoscale devices changes abruptly and the correlation functions arising from coupled mode effects are improperly evaluated in the current and charge density calculations, it becomes difficult to solve nonequilibrium Green’s function equations self-consistently with Poisson’s equation because discrepancies appear in the charge distribution. To avoid this complication, two- and three-dimensional structures with a constant or a slightly varying confinement are often considered in the NEGF coupled mode space approach. We present a rigorous derivation of the method starting from the definition of the Green’s function and its expansion in a coupled mode space, where current and charge density equations fully account for the coupling effects. Excellent agreement with real space calculation demonstrates the strength of the method and its applicability to the simulation of two- and three-dimensional nanoscale metal-oxide-semiconductor field-effect transistors with abruptly flared-out source/drain contacts in the ballistic limit.