Reliability of semiconductor devices for submarine-cable systems

The Bell System initiated development of semiconductor devices for use in broad-band repeatered coaxial submarine-cable telephone systems in the early 1960´s. Development of such devices has continued at varying levels of activity to this date. Now, over a decade later, more than five years of successful operational life have elapsed on the first installed system; one transatlantic system is in operation and several more of equal or greater length are in various stages of construction. Laboratory-like life validation tests have been conducted on all the devices for use in these systems, and up to seven years of history have elapsed on groups of 100 devices representative of the five varieties used in the first system. No failures have been observed either in service or in life validation tests, thereby confirming a FIT rate of better than five. Moreover, variables data for the representative samples aged for seven years indicate that the confirmed FIT rate may be extremely conservative. Silicon diodes aged below breakdown are more stable than the life-test system designed to evaluate their reliability. Silicon diodes aged in breakdown show a linear drift in leakage current which is so small that it is not detectable except with an ultrasensitive test system. Germanium transistors evaluated over this same period show that the change in current gain they exhibit will be substantially less than the goal established for them at the inception of development. Silicon transistors being developed for a new higher capacity system show promise of exceeding the stability of the more extensively evaluated germanium devices. These results have broader implications than their relevance to submarine-cable system performance since they indicate that the reliability which has long been considered to be inherent in semiconductor devices not only has been demonstrably achieved in quantitative terms but can be considered to be designable for future system applications.