Analysis of parametric drift of a MESFET-based GaAs MMIC due to 125 degrees C storage

The authors point out that microwave attenuation drifted significantly for two out of twenty-nine GaAs MESFET-based variable voltage attenuator MMICs (VVAs) after ten weeks of storage at temperatures of either 125 degrees C or 150 degrees C. Poststorage analysis of VVAs which drifted and others that were stable showed that the drift was due to a die-wide shift of the FET I/sub DS/-V/sub G/ characteristics toward greater pinchoff voltage. The effect does not appear to be due to contamination. Experiments with a different MMIC circuit produced with the same process found drifts in some FET I/sub DS/-V/sub G/ characteristics similar to those inferred from the RF attenuation changes of the VVAs. The Schottky barrier is stable, and not responsible for the drift. The drift is due to a change in the gate-controlled portion of the channel. The ohmic contacts in FETs which drifted were misaligned with respect to the gates, suggesting an interaction between the channels and ohmic contacts. The experiments also show that drift in this technology has been eliminated by improved lithography.<
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