Progressive Failure Mechanisms of a Commercial Silicon Diode

Two forms of progressive degradation of a commercial silicon signal diode have been stuided as a function of various stress and measurement variables. One mode results from reverse bias at high temperature, and the second is caused by prolonged stressing with high forward current. For both modes the rate of degradation can be greatly increased by raising the ambient temperature; and therefore, accelerated testing is useful in both cases. The maximum acceleration is limited only by the onset of catastrophic failure mechanisms, which supersede the progressive mechanisms at sufficiently high stress levels. The reverse current characteristic is the principal degrading parameter for both modes. Models are presented to relate the stress-induced increases in IR to physical and chemical changes taking place in the diode. The reverse bias degradation mode may be readily explained by a model based on the drift of impurity ions in the silicon oxide and the resultant inversion of the epitaxial n layer. The actual source of the reverse current is space charge generation in the stress-induced inversion layer. There is a well-defined time constant for the degradation rate, and its temperature dependence is found both experimentally and theoretically to be of the Arrihenius type. The excess reverse current resulting from forward bias stressing may be attributed to a form of surface breakdown caused by a stress-induced accumulation layer on the n-type epitaxial material.