Development and Experimental Verification of a Two-Dimensional Numerical Model of Piezoelectrically Induced Threshold Voltage Shifts in GaAs MESFET’s

Abstract

The results of a combined experimental and analytical investigation of the effects of mechanical stress on dc electrical parameters, particularly threshold voltage, in MESFET’s are reported. The theoretical aspect of this study involves a two-dimensional finite element simulation of the same device structure on which measurements were made. In contrast with an approximate analytical calculation reported in the literature in which the stress concentrations that occur at the gate edges were represented by concentrated line forces acting in the plane of the substrate surface, 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. Accounting for the two-dimensional nature of the overlayer yields piezoelectric charge distributions that differ from those predicted using the more approximate concentrated force model. The experimental portion of this study involves measurement of dc parameters of devices using the application of external mechanical loads. These loads are intended to simulate mechanical stresses that arise during device processing. By introducing this stress without any additional thermal processing, the impact of residual stresses via the piezoelectric effect on parameters such as threshold voltage can be examined separately from other effects, such as stress-enhanced diffusion. © 1988 IEEE