Some Failure Modes of Double Diffused Silicon Mesa Transistors

THIs paper discusses two different failure modes (bulk failure and surface failure) which have been observed on double diffused silicon n-p-n mesa transistors. (a) In the presence of bare copper in an oxygen free ambient, power aging degrades the emitter parameters and gain. Only partial recovery can be achieved by etching into the bulk silicon or by heating the device at 300°C (bulk failure). (b) In gold plated and/or oxygen backfilled cans, soft, loopy reverse junction characteristics develop under both temperature and power agings, first on the collector and later on the emitter. Both junctions. recover completely upon opening the can and drying the transistor surface (surface failure). Surface failure is caused by water adsorption over the surface of the silicon wafer. Experimental evidence, including aging experiments in atomic hydrogen, is presented to demonstrate that the bulk failure is caused by copper contamination in the bulk silicon. Copper is transferred from the can to the wafer via a volatile hydride. It diffuses into the silicon and becomes electrically active during power aging. Qualitative explanations are offered for both failure modes. Surface failure is due to surface states introduced by the adsorbed water and/or ionic conduction. In order to explain bulk failure, the solubility and precipitation of copper is examined over the transistor profile and the effect of field on the migration of copper in silicon is taken into account.