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Influence of surface states on the reverse and noise properties of silicon power diodes

This contribution investigates transient degradation of reverse characteristics of diodes by means of a noise measurement. This effect appears immediately after external heating or after a long time on-state polarisation of diodes (without a significant temperature growth of the device in this case). Simultaneously with the reverse characteristics degradation, the noise power measured under a low voltage DC reverse bias is influenced. The first possible cause of these effects is connected with so called slow surface states (SSS). The SSS are caused by the presence of material process induced defects in the region of a p-n junction surface termination. SSS have fundamental impact on the reverse properties of diodes and their low frequency noise behaviour. The second cause is connected with so called volume structural defects (VSD). Their origin can be ‘genetic’ (e.g. the presence of imperfection inside a silicon crystal) or they can be induced during technological processing. These defects are not repairable and under the reverse bias they will form so called ‘hot spots’ that is, places with a local high current density. Rapid and operative measurements of the noise power can reveal latent instabilities of reverse characteristics invisible during a standard inspection process during the production.