On the extrapolation of accelerated stress conditions to normal stress conditions of germanium transistors

Accelerated stressing has become a popular method of evaluating high reliability devices in short periods of testing time. However, the convenience of accelerated testing should not be the prime factor for use. Tl he modes of failure generated by normal (within rating) and accelerated (exceeding rating) stressing should be compared before failure rates created bv accelerated stressing are accepted as valid. It has been found for moisture gettered PNP germanium mesa transistors that the predominant modes of failure created during accelerated stressing are different from the predominant modes of failure created during normal stressing. During accelerated unpowered thermal stressing, parameter changes are exactly opposite to parameter changes during normal unpowered thermal stressing. The direction and irreversibility of parameter changes, the dependence of amount of parameter change on the stabilization bake temperature and internal package vapor pressure, anid a good fit of hFiE decay to Wallmark´s equation: 1/hFE = C1 e-Cs/T t1/3+C3, all point to oxidation of the germanium surface. The activation energy as determined from the above equation is discussed. Oxidation of the germanium surface is negligible at temperatures less than stabilization bake temperatures. Accelerated powering produced several predominant failure modes not observed during normal power stressing. Electrical overstressilng characterized by lead wire melting and junction shortinig were first-orler failures. Failures by junction shorting through the bulk vere reverse collector voltage dependent as well as power dependent. Tlhe failures were attributed to operation in a negative resistance mode.