Assessment of approximations in nonequilibrium Green's function theory

A nonequilibrium Green's function (NEGF) method for stationary carrier dynamics in open semiconductor nanodevices is presented that includes all relevant incoherent scattering mechanisms. A consistent lead model is developed that ensures all observables to reflect intrinsic device properties. By restricting the charge self-consistent calculations to vertical transport through heterostructures, the Green's functions and self-energies can be determined very accurately. This allows us to assess many commonly used approximations, such as ballistic leads, decoupling of Dyson's and Keldysh's equations, truncated or momentum-averaged self-energies, and local self-energies in the NEGF formalism in detail, and to study limiting cases such as diffusive transport in resistors. The comparison of exact and approximated NEGF calculations illustrates the physical implications and validity of common approximations and suggests numerically efficient simplifications.