Failure analysis for RF characteristics of GaAs MESFETs

The effect of thermal stress on the noise degradation of GaAs MESFETs was investigated. Minimum noise figure, associated gain, scattering parameters, and capacitance-voltage profiles were measured during the tests, and the electro-chemical properties after thermal stress were analyzed by means of Auger electron spectroscopy, X-ray diffractometery, cross-sectional transmission electron microscopy, and capacitance-voltage measurements. The RF failure mode consists of an increase of the minimum noise figure and a decrease of associated gain of the FETs. The extracted equivalent circuit elements from measured scattering parameters were used to evaluate the influence of each parameter on the noise degradation. The parametric estimation showed that the noise degradation was mainly attributed to the decrease of a.c. transconductance. From the C-V analysis, we found that the decrease of a.c. transconductance was caused by the decrease of effective carrier concentration. From the electro-chemical analysis, the decrease in effective carrier concentration was resulted from the gate-sinking by the thermally activated interdiffusion between the gate metal and the GaAs channel layer. Therefore, the thermally activated carrier compensation by Ga vacancies in the channel is proposed to be the main failure mechanism for noise degradation of GaAs MESFETs.