Defect-related generation-recombination region in heterojunction diodes

Abstract

A numerical analysis is presented of semiconductor heterojunctions containing deep defect levels, in order to study the mechanism of generation or recombination of electron-hole pairs induced by thermal or optical transitions involving deep defect energy levels. The final aim is to determine the width of the effective generation or recombination region Wgr and compare to the width of the space-charge region W. Numerical solution of Poisson’s equation and the current-transport equations yield the electrical potential and the quasi-Fermi levels for electrons, holes, and occupied deep defects. The resulting transition rates yield the net generation rate of electron-hole pairs as function of position x. An application is made to an InGaAs/InP heterojunction with a midgap level on the InGaAs side. Results for a Si homojunction with a deep acceptor level at Ec−0.54 eV are given for comparison. The applications show that in all cases of reverse bias Wgr is smaller than W, reaching for some parameters describing the heterojunction system a value of 1/3 for the ratio Wgr/W. Usually the two widths are assumed to be equal. The physical origin of this reduction is the existence of competing recombination processes due to free-carrier tails extending from the neutral regions into the space-charge region. In heterojunction structures, this effect is even enhanced in case of accumulation of free carriers due to conduction- or valence-band discontinuities at the junction plane. It is shown that if one considers defects states only in a finite region close to the junction, this may yield marked differences in the value of the resulting current as compared to the homojunction case.