DISCRETE CONDUCTANCE FLUCTUATIONS IN SILICON EMITTER JUNCTIONS DUE TO DEFECT CLUSTERING AND EVIDENCE FOR STRUCTURAL-CHANGES BY HIGH-ENERGY ELECTRON-IRRADIATION AND ANNEALING

Observations of discrete conductance fluctuations are reported at voltages well below the breakdown voltage in selected reverse-biased p+-n++ base-emitter junctions originating from gate turn-off thyristors. The occurrence of the phenomenon is attributed to the presence of defect clusters at the p-n junctions. The defect clusters introduce field confinements which activate tunneling processes that would not otherwise be present in these nonabrupt p-n junctions. The fluctuating reverse current was only observed in voltage and temperature regions where the total reverse current was influenced by tunneling-related conduction mechanisms. The experimental observations concerning the voltage and temperature dependences of the fluctuation amplitude and rate deviate from earlier reports on decisive points. Both the amplitude and the switching rate of the observed fluctuations were unstable in time and influenced by the measurement procedure itself. This instability is attributed to small structural changes of the defect clusters. Furthermore, the unstable behavior of the defect clusters also influences the static reverse current-voltage characteristic. Distinct changes were found in the static reverse current-voltage characteristics of selected samples due to high-energy electron irradiation and annealing at 200-degrees-C. A clearly increased uniformity of the reverse current-voltage characteristics between the gate-cathode junctions of gate-turn off thyristors was also found as a result of electron irradiation. The changes observed are interpreted as evidence of structural changes of the defect clusters.