Abstract :
NPN silicon transistors exhibit changes in common emitter current gain when subjected to various aging and environmental treatments, among which are exposure to radiation, effects of processes and gas environment, and voltage and temperature stressing. These changes were studied using a device specified over a large current range. Most of these effects are due to surface states, and can be reversed. The effects of metallic contamination on the surface were studied by preparing special samples of transistors with and without aluminum metallizing and comparing their performance after aging in different ambients under various cycles of exposure and treatment. After the current gain has degraded by particular stress treatments, it can be restored to its original value by heat treatment in specific atmospheres. The time required varies from 1 hour to 2 or 3 days. Furthermore, heat treatment in non-oxidizing ambients can again decrease the current gain. Additional experimental studies demonstrate the effects on current gain of biasing the emitter junction. A model fitting the experimental data is presented. This model discusses the exchange of charge between the oxide surface and the interface between the silicon and the oxide. The role of oxygen ions and metallic ions as the species involved in the charge exchange is postulated and discussed.
