DEGRADATION MECHANISM OF GAAS-MESFETS

The degradation mechanism of X-band low-noise GaAs MESFET's is examined to obtain meaningful information on a common mode of failure. The devices tested have a half-micrometer gate (Au/ Mo) and source and drain ohmic contacts (Au/Ni/Au-Ge). Zero bias drain conductance gDo is considered as a representative parameter for degradation during aging. The major failure mode is an increase in series resistance of the ohmic contacts. The amount of degradation, decrease in gDo, is proportional to the square root of aging time, and accompanied by an increase in minimum noise figure Fmin. A degradation model based on the formation of a high-resistance layer between the ohmic metals and GaAs crystal by a diffusion reaction mechanism is proposed, resulting in excellent agreement between calculated and experimental results. Using ion-microspectroscopy analysis (IMA), diffusion of Ni into GaAs crystal is revealed. Mean time to failure (MTTF) is estimated to be 107–108 h at channel temperature of 80°C with an increase in Fmin of 0.5 dB as failure criterion. Copyright © 1979 by The Institute of Electrical and Electronics Engineers, Inc.