A novel MSI circuit and reliability test method has been developed to measure the probability of infant failure in InGaP/GaAs HBTs. This circuit enables simultaneous reliability testing of 200 HBTs per circuit, and has been used routinely to test 12,800 HBTs/wafer (and sometimes 100, 000HBTs/wafer) to measure the probability of infant failure Pqf on each wafer. We have applied this MSI reliability test method to more than 100 production HBT IC wafers and many experimental wafers. With these tests, we have identified a new HBT failure mode, for which a very small fraction of HBTs (Pqf = 10 -3 to 10 -4 ) fail by beta degradation much sooner than the main HBT population. Moreover, the probability of infant failure Pqf is proportional to the substrate EPD, with a constant of proportionality approximately equal to the emitter area. This observation is consistent with a model where any HBT whose e-b junction is pierced by a substrate dislocation will suffer beta degradation much sooner than its companions. This data has motivated Agilent to procure substrates with the lowest possible EPD for the manufacture of high complexity HBT ICs with sufficient reliability for use in Agilent instrument products.