Wafer-level reliability assessment of SiGe NPN HBTs after high temperature electrical operation

The decrease of the common emitter current gain, h/sub FE/, of SiGe NPN HBTs after electrical operation has been investigated in detail. Long term (package-level) experiments at the nominal collector current I/sub c/ and high temperature showed that the observed h/sub FE/-degradation is due to an increase of the base current I/sub b/. This increase of I/sub b/ is caused by an enhanced recombination. Short term (wafer-level) experiments, employing higher collector stress current (I/sub c/-stress), showed that I/sub c/-stress is very crucial for the observed parameter shift. Nevertheless the higher collector stress current I/sub c/-stress originates no additional failure mechanism. The experimental data from wafer-level experiments fit very well a ln(/spl tau/) versus I/sub c/-stress correlation. Furthermore the expected h/sub FE/-degradation from package-level tests at nominal collector current I/sub c/ can be extrapolated very accurately within a prediction interval of 95% applying this correlation. Using the discovered extrapolation model it was found that the current gain drift due to I/sub c/-stress acceleration dominates the one caused by temperature acceleration.