Abstract
The orientation dependence of the threshold voltage V T of GaAs MESFETs before and after passivation with a SiN x overlayer was investigated. [001] and [010] oriented FETs have the same V T which does not change after passivation. [011] and [01̄1] oriented FETs show opposite V T shifts relative to the [001] FETs. The sign of the shift is reversed by passivation. The measured stresses in the gate metal and dielectric overlayer are both tensile. A model of the piezoelectrically induced charges is extended to include the effects of both the gate metal and dielectric overlayer. Using the measured stress values, a good conformity between experimental results and model calculations is obtained. In particular it is demonstrated that the same stress in both layers produces piezoelectric charges of opposite sign in the MESFET channel region.