ABSTRACT
Atomic-scale defects are discrete in nature, as opposed to the continuum material description used in technology computer aided design models. This chapter reviews some aspects of the state-of-the-art modeling of currents from a quantum-mechanical atomistic perspective. It focuses on developments of relevance for more traditional semiconductor materials and device types, although the principles apply very well also to more exotic device designs based on molecular junctions, nanotubes, graphene, and so on. Modeling of nanoscale structures such as nanowires, nanotubes, graphene devices, molecular junctions, and ultrathin dielectric junctions has evolved over the last decade into a position where several thousand atoms can now be simulated with atomistic, quantum-mechanical methods, using methods ranging from density-functional theory (DFT) to tight-binding models. A common concern about DFT is the issue related to obtaining a correct description of the band gap of semiconductors. For some applications, this is not really an issue—geometrical properties and total energies are often well represented by DFT.
