Base Current Investigation of the Long-term Reliability of GaAs-Based HBTs

R.E. Welser, M. Chaplin, C.R. Lutz. M/ {am
Kopin Corporation, 695 Myles Standish Blvd., Taunton, MA  02780
A. Gupta, B. Veasel, A. Ezis
Northrup Grummna, 1212 Winterson & W. Nursery Roads, Linthicum, MD  21090

We observe a 5x increase in the time to failure of Al0.28Ga0.72AS/GaAs HBTs with lower Vbe turn-on voltages.  Degradation in dc current gain is used as the criteria for failure analysis.  Standard four-finger 1.7 mm x 19.7 mm devices both stressed and tested at a junction temperature of 260oC and a collector current density of 48.5 kA/cm2 exhibit a MTTF under 400 hours.  Devices from wafers of ideentical structure except for a 53 mV lower turn-on voltage (@ Jc=1.78 A/cm2) have a MTTF in excess of 2000 hours.  These results are simulated using a simple Gummel plot model for the various base current components in a well passivated device.  An expression consistent with a recombination enhanced defect formation process is used to quantify an increase in trap density within the emitter-base depletion region.  The general features of previously reported reliability results, such as the observed 10x difference in MTTF between AlGaAs and InGaP emitter HBTs, can also be simulated using the model discussed in this work.

 

 

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