Transistor cutoff characteristics in oils

In certain ocean engineering applications, such as pressure-tolerant electronic (PTE) systems, semiconductor devices are exposed to dielectric oils containing a host of contaminants, including dissolved ionic species. The induced electric-field drift of these charged species and their ensuing accumulation near the surfaces of the semiconductor materials has been postulated as a possible failure mechanism. To ascertain the magnitude and limits of such an ion-induced failure mechanism, a technique utilizing the characteristic of the collector-base cutoff current was developed. This characteristic lends itself to evaluating the deleterious effect of oil-soluble ions enveloping semiconductor devices. With this technique it was found that the cutoff current of unpassivated semiconductor junctions increases with the concentration of dissolved ionic species. On the other hand, contaminant induced changes in the cutoff current characteristic of passivated devices depend on collector doping. For p-type collector regions, the cutoff current increases when the device is operated in oil contaminated only with a substantial concentration of ionic species, while the cutoff current of devices with n-type collector regions is unaffected by operation in even a concentrated solution of ions dissolved in oil. In addition, washing surfaces of devices previously exposed to contaminated oils was found to restore the cutoff current to its pure-oil steady-state value. Limited testing under pressure indicates that the same results can be anticipated in PTE systems. The findings and conclusions give added credence to the feasibility of achieving the specific ocean-engineering goal of operating solid-state devices immersed in an oil.