Second breakdown and degradation in germanium alloy junctions

N-P-N germanium alloy (N⁺P) junctions were found to be much more susceptible to damage by high voltage pulses than were P-N-P transistor (P⁺N) junctions. The damage manifested itself as a collapse of the reverse diode characteristics and physical junction degradation. A failure mechanism involving both surface breakdown and second breakdown has been shown to account in part for the N⁺P junction´s high susceptibility to damage. Surface treatments commonly used in device manufacture leave the P-type surface with accumulation layers and surface breakdown sites. The enhanced current concentrations at the surface breakdown sites lead to local heating with a consequent low second breakdown threshold. Minority carrier injection from the contact to the P-type base wafer was also found to contribute to the low second breakdown threshold of the N⁺P junction. The use of a pure indium clad base tab on N-P-N transistors resulted in a contact that suppressed the minority carrier injection and raised the second breakdown threshold.