Reliability of InGaP-Emitter HBTs
Bob Yeats, Paul Chandler, Morgan Culver, Don D'Avanzo, Gilbert Essilfie, Craig Hutchinson, David Kuhn, Tom Low, Tim Shirley, Sunil Thomas, Wes Whiteley
Agilent Technologies, 1412 Fountaingrove Parkway, Santa Rosa, CA 95403
Various reliability results are discussed for Agilent's InGaP-emitter HBTs. The MTTF distribution for 30 wafers, stressed at the severe stress of TJ = 334oC and JE = 1.8 mA/mm2, has a median value of 1159 h. Activation energy (EA) and current density acceleration exponent (nJ) have been measured on discrete HBTs for 6 wafers in the early-to-fail part of the MTTF distribution. We consistently found EA = 1.2 eV. while nJ could be determined only crudely (~1.2) due to the limited range of current density used. Wafers with intentional lattice mismatch in the InGaP layer were found to be consistent with the same (nominally lattice matched) MTTF distributions, although a wafer with the largest Ga-rich mismatch had among the best lifetimes (85th percentile). The failure mode that predominates in our devices is sudden b degradation to b = 1. The large base currents triggered by device failure enable MSI-level reliability test circuits to be designed that are sensitive to failure of a single transistor. Such circuits are being used to explore the threat of infant failures in large circuits. Base current appears to be a more general indicator than emitter current for the MTTF current acceleration factor, as the large base current acceleration exponent of nJB = 1.8.