Development and experimental verification of a two-dimensional numerical model of piezoelectrically induced threshold voltage shifts in GaAs MESFETs

The results of a combined experimental and analytical investigation of the effects of mechanical stress on DC electrical parameters, particularly threshold voltage, in MESFETs are reported. The theoretical aspect of this study involves a two-dimensional finite-element simulation of the device structure on which measurements were made. The substrate stresses and resultant piezoelectric charge distributions calculated in this study take into account the two-dimensional nature of the geometry of the gate. The experimental portion of this study involves measurement of DC parameters of devices using external mechanical loads that simulate mechanical stresses that arise during device processing. Measurements under applied loads of both signs and on devices of two different orientations confirm the existence of a piezoelectrically induced threshold voltage shift. A comparison between the approximate line load method of modeling substrate stress fields, and the finite-element method used in this study shows that the piezoelectric charge densities predicted by two models are substantially different. This results from the fact that the simplifying assumptions used to construct the line load model are inappropriate for accurately determining stress fields beneath micrometer and submicrometer gates. Good agreement was obtained between measured threshold voltage shifts and those predicted by the finite-element method model. The results show the need for accurate modeling of mechanical stresses when attempting to model piezoelectric effects.<
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