Carrier Transport

Abstract

AbstractThe two main mechanisms responsible for the charge carrier transport in semiconductors are diffusion and drift. Diffusion is caused by a spatial non-uniformity of the charge carrier concentration, and drift is induced by an electric field. The two mechanisms are quantified in terms of the flux density. The diffusion flux density is proportional to the concentration gradient, while the drift flux density is proportional to the product of the concentration and the electric field. The respective proportionality constants are called the diffusion coefficient and the mobility of electrons and holes. Einstein’s relation is derived, which states that the ratio of these two parameters equals the thermal voltage. The drift of charge carriers in the presence of a magnetic field results in their deflection in the direction perpendicular to both the electric and magnetic fields. This produces the so-called Hall voltage across a semiconductor sample in the direction perpendicular to the current flow. The Hall voltage measurements allow one to determine the sign and the concentration of the majority carriers. Continuity equation is derived, which describes the charge flow due to both drift and diffusion when generation and recombination processes are operative.KeywordsFlux densityElectric current densityDiffusionDriftConductivityResistivityPhotoconductivityHall effectMobilityDiffusion coefficientEinstein’s relationContinuity equation