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

The decrease of the common emitter current gain, h_FE, of SiGe NPN HBTs after electrical operation has been investigated in detail. Long term (package-level) experiments at the nominal collector current I_c and high temperature showed that the observed h_FE-degradation is due to an increase of the base current I_b. This increase of I_b is caused by an enhanced recombination. Short term (wafer-level) experiments, employing higher collector stress current (I_c-stress), showed that I_c-stress is very crucial for the observed parameter shift. Nevertheless the higher collector stress current I_c-stress originates no additional failure mechanism. The experimental data from wafer-level experiments fit very well a ln(τ) versus I_c-stress correlation. Furthermore the expected h_FE-degradation from package-level tests at nominal collector current I_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_c-stress acceleration dominates the one caused by temperature acceleration.