1 - Introduction

Summary

Nanostructures are generally regarded as ideal systems for the study of electronic transport. What does this simple statement mean?

First, consider transport in large, macroscopic systems. In bulk materials and devices, transport has been well described via the Boltzmann transport equation or similar kinetic equation approaches. The validity of this approach is based on the following set of assumptions: (i) scattering processes are local and occur at a single point in space; (ii) the scattering is instantaneous (local) in time; (iii) the scattering is very weak and the fields are low, such that these two quantities form separate perturbations on the equilibrium system; (iv) the time scale is such that only events that are slow compared to the mean free time between collisions are of interest. In short, one is dealing with structures in which the potentials vary slowly on both the spatial scale of the electron thermal wavelength (to be defined below) and the temporal scale of the scattering processes.

Since the late 1960s and early 1970s, researchers have observed quantum effects due to confinement of carriers at surfaces and interfaces, for example along the Si/SiO₂ interface, or in heterostructure systems formed between lattice-matched semiconductors. In such systems, it is still possible to separate the motion of carriers parallel to the surface or interface, from the quantized motion perpendicular, and describe motion semiclassically in the unconstrained directions.