Degradation mechanism of GaAs MESFET´s

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 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 , is proportional to the square root of aging time, and accompanied by an increase in minimum noise figure . 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 10⁷-10⁸h at channel temperature of 80°C with an increase in of 0.5 dB as failure criterion.