Numerical simulation of time resolved charge transport in semiconductor structures for electronic devices

Excess charge carrier transport and relaxation in semiconductor layered structures have been numerically simulated by a finite-difference method. The standard mathematical model of charge transport in semiconductors has been used, consisting of two continuity, two diffusion and two drift equations and a Poisson equation involving electron, hole concentrations and potential. The relaxation process is described by means of Shockley-Read-Hall recombination statistics. The equations of the model are solved in a one-dimensional space domain and the time domain. The simulation program can be used to deduce semiconductor parameters like bulk lifetime, surface recombination velocity, diffusion coefficients and mobilities of electrons and holes from measurements of photoconductance decay. A technique for a direct measurement of the recombination velocity at a wafer's surface or interface is presented. It is based on the measurement of the initial photoconductance decay after laser pulse excitation of excess charge carriers in a very shallow layer at the surface or interface. Under conditions to be fulfilled for a correct measurement, the initial decay process is dominated by recombination at the surface and the decay of the measured curve depends almost exclusively on the surface recombination velocity. In this case the simulation program is necessary to interpret the measurement data.