Studies on dynamic avalanche and current filaments in high-voltage diodes

When bipolar semiconductor devices driven by a high current are turned off with a sufficiently high current rate d I / d t , the existence of free charge carriers can lead to charge carrier multiplication at device voltages far below the static breakdown voltage. Numerical studies on the turn-off behaviour of high-voltage power diodes are used to analyse the destabilisation of the homogeneous current-density distribution and the evolution of current-density filaments under isothermal conditions. It is shown that depending on the d I / d t rate the homogeneous current flow can be destabilised by a fluctuation with either the longest possible wavelength that depends upon the sample length or a well-defined period that is independent of the sample length. While the first type of destabilisation leads to the appearance of a travelling filament, the second one results in the development of a quasi-stationary filament.