DYNAMICS OF STOCHASTICALLY INDUCED AND SPATIALLY INHOMOGENEOUS IMPURITY BREAKDOWN IN SEMICONDUCTORS

Impact ionization of trapped carriers in high electric fields at Heliu m temperatures can lead to threshold switching, i.e., switching of a s emiconductor sample from a nearly insulating state to a high conductin g steady state due to an applied voltage that exceeds the threshold va lue for breakdown. This phenomenon is investigated theoretically based on a set of partial differential equations containing the relevant ca rrier transport mechanisms. In a first analysis the impact of fluctuat ions on the breakdown process within the framework of a master equatio n description of spatially homogeneous generation-recombination kineti cs is computed. Then, taking into account spatially inhomogeneous long itudinal modes, we find that threshold switching constitutes a three-s tage process, consisting of a stage of front creation and propagation, a stage dominated by a seesaw-like mode, and one in which impact ioni zation occurs nearly homogeneously throughout the sample.