Accelerated ageing of IMPATT diodes

An investigation conducted to determine suitable reliability test methods for IMPATT diodes has shown that elevated temperature accelerated their normal degradation in a predictable way only when the diodes were stressed with normal d.c. power applied and controlled in a constant-voltage mode. Normal degradation was manifested initially by an increase in thermal resistance and ultimately by short-circuits, both of which were caused by the progressive interaction of the constituents of the contact metallizations. Initial instabilities and infant mortalities had to be eliminated by burning-in the diodes before overstress results could be related to long-term wearout. Thereafter, simple d.c. was adequate to assess life expectancy because the changes in thermal resistance could be used to estimate consequent changes in r.f. performance.