Degradation mechanism of GaAs MESFET's

Abstract

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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g_{D0}</tex> 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 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g_{D0}</tex> , is proportional to the square root of aging time, and accompanied by an increase in minimum noise figure <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F_{\min}</tex> . 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 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> h at channel temperature of 80°C with an increase in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F_{\min}</tex> of 0.5 dB as failure criterion.