Premature failure in Pt-GaAs IMPATT´s—Recombination-assisted diffusion as a failure mechanism

We have recently observed an aging anomaly in platinum-- GaAs IMPATT diodes. The effect is observed during temperature stress aging of diodes operating in avalanche. At temperatures above about 300°C, the devices follow a log-normal failure distribution with an activation energy of 1.6 eV. At lower temperatures, however, a change in activation energy to the range of 0.2 to 0.4 eV is observed. This leads to a much lower median life than would be predicted from the high-temperature data. The effect is not observed under temperature stress alone. The failures are caused by diffusion of defects from the platinum-GaAs interface into the diode during operation. We have analyzed the energy levels in the bandgap which are associated with the defects. Analysis of the diffusion of the defects and comparison with some earlier work in III-V compounds lead to the conclusion that the defect motion is enhanced by recombination through the energy levels associated with the defects. At temperatures above 300 °C, defect motion is thermally activated while below 300°C, the kinetics of recombination-enhanced diffusion dominate. This results in the apparent anomaly when high-temperature aging data are compared with the behavior of the device at lower temperatures. This failure mechanism renders thick platinum-GaAs an unsuitable combination for IMPATT structures. The effect can be avoided, however, by the use of a thin platinum layer in conjunction with a diffusion barrier such as titanium. Recombination-enhanced diffusion is a potential failure mechanism for many devices. The principle requirements are a population of diffusable simple defects and a source of recombination energy, e.g., injection currents or avalanche conditions.