Avalanche multiplication and breakdown in wide bandgap semiconductors

A Monte Carlo (MC) model employing three conduction band (CB) and valence band (VB) valleys described analytically is shown to reproduce successfully results from multiplication measurements performed on AIxGa1-xAs (x = 0.0 to 0.3) p-i-n devices of nominal avalanching thicknesses, W, from 0.5μm down to 0.05μm. The dead space d, defined as the distance carriers travel before gaining sufficient energy to initiate ionisation, is found to be insignificant in thick structures but to occupy a significant portion of the high field region in thinner devices. Also, as the field is increased d is reduced and carriers are accelerated to energies greater than their steady state value resulting in an ionisation overshoot. These results confirm that for the AIxGa1-xAs compositions investigated here, non-local effects become more important as the device becomes thinner. Multiplication measurements, carried out on Ga0.52IN0.48P devices (in which W ranges from nominally 2μm down to the width of a p-n junction) have been analysed assuming that ionisation depends only on the local electric field. The ionisation coefficients deduced are found to fall with decreasing W. When compared to GaAs structures of the same dimensions the total breakdown voltage VT in Ga0.52IN0.48P is found to be 1.8-1.9 times higher across the range of W studied. A simple MC model using a single CB and VB valley was used to interpret these results and showed the existence of a dead space and ionisation overshoot that is similar to, but less prominent than that seen in GaAs. Multiplication and breakdown measurements were conducted on GaAs p-i-n devices (W = 0.1μm to 1.0μm) across the range 20K to 500K, and on Ga0.52IN0.48P p-i-n devices (W = 0.2μm to 1.0μm) across the range 20K to 600K. Ga0.52IN0.48P is shown to be more temperature stable than GaAs and the deduced ionisation coefficients for both material systems are shown to decrease with temperature. Using the more economical single valley model, the ionisation overshoot is shown to decrease, whilst the variation in the ionisation path length is shown to increase with temperature. (author)