Transfer fields and Langevin forces for the noise analysis in diffusive nanodevices within the nonequilibrium Green’s function approach

Advanced applications based on semiconductor nanostructures require accurate modeling tools for the description of coherent and dissipative quantum processes, but, crucially, also microscopic fluctuations and their propagation to the contacts. Since conduction and fluctuations are intimately related by the fluctuation-dissipation theorem, the analysis of carrier transport and noise properties should be addressed at the same level of approximation to obtain a complete and unambiguous understanding of the quantum system. Starting from a quantum transport model based on the nonequilibrium Green's function formalism, we identify a set of microscopic noise sources in terms of fundamental current and charge fluctuations in the presence of dissipative processes. The noise sources are added to the relevant conservation equations, and the power spectra of the short-circuit current fluctuations are evaluated through a linear perturbation theory equivalent to the classical impedance-field method for noise analysis. Numerical examples with textbook structures are shown to establish a connection with semiclassical noise theories. Application to a resonant tunneling diode demonstrates the potential of the proposed model as a tool to probe the electron kinetics in highly nanostructured devices. The present work concerns fluctuations associated with intraband scattering processes. The extension to number fluctuations due to interband scattering could provide information on the technological quality and physical origin of degradation mechanisms in state-of-the-art optoelectronic devices such as type-II superlattice photodetectors.