Bulk and contact effects in p+-SI-n+ semi-insulating GaAs structures

Abstract

Current–voltage characteristics, free- and trapped-carrier concentrations, electric field, and excess carrier lifetime contours are given for a p+-SI-n+ GaAs structure. This analysis is done by numerical simulation of the two carrier drift-diffusion model of conduction. The dependence of the deep trap parameters and of the semi-insulating (SI) thickness on the J-V characteristic is presented. Particular emphasis will be put on the study of the equivalent resistivity and electric-field overshoots inside the structure. It is shown that, except for very thick samples, contact and near contact effects on the low-voltage electrical conduction mechanism are all important. It is then necessary to take into account these effects for a precise and exact interpretation of transport in a p+-SI-n+ structure. When the deep trap density Nt is high (Nt≫ni), as is the case for compensated (SI) GaAs, the equivalent low current resistivity ρ is higher than the equilibrium (unperturbed bulk) resistivity ρe. The nature, electron trap or hole trap, of the deep level affects the equivalent resistivity at intermediate applied voltage. When the density of deep traps is low, ρ decreases from values higher than ρe to values lower than ρe as the sample thickness increases. Only in the limit of very thick samples, and then regardless of the trap concentration, does the equivalent resistivity tend asymptotically towards its equilibrium value.