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

Abstract

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 same device structure on which measurements were made. In contrast with an approximate analytical calculation reported in the literature in which the stress concentrations which occur at the gate edges were represented by concentrated fine 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 during the application of external mechanical loads. These loads are intended to simulate mechanical stresses which 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. It is found that the piezoelectric effect can account for most of the anomalous shift in threshold voltage observed in real GaAs devices.

Broadband Distortion Cancellation Technique in GaAs HJFET and GaN FET Cascode Amplifier

“…GaAs substrate has the elastic constants of ∼85 GPa for Young's modulus. The WSi used for the gatemetal and SiO 2 for passivation-film have ∼620 and 70 GPa for Young's modulus, respectively [14]. In this device structure, the piezoelectric charge induced in the FET channel region by stress was intentionally used to optimize the gm-profile.…”

A new two-dimensional model for the potential distribution of short gate-length MESFET's and its applications

“…However, the gate-controlling capability will be reduced by the penetration of the electric field from the sidewall at both sides of the gate, and the threshold voltage of a short gate-length device will be influenced by the drain bias. The short gate-length effect can be exactly analyzed by numerical simulation, based on a full set of semiconductor device equations with reasonable boundary conditions [ 11- [3]. However, the numerical simulation is limited by the computation time and is difficult to use in circuit analysis.…”

Synchrotron X-ray topographic analysis of the impact of processing steps on the fabrication of AlGaAs/InGaAs p-HEMT's

“…The mechanical stresses imposed on the substrate by such metal or dielectric overlayers lead to large regions of uncontrollable piezoelectric charge in the active regions of a device [2]- [5] and have a serious impact on the electrical performance of these devices, e.g., stress-induced threshold voltage shifts of the order of 500 mV in some situations [6]. Furthermore, in the event of these overlayers being responsible for defect generation within the substrate, it is well known that the proximity of these crystalline defects or dislocations is correlated with changes in the electrical characteristics of 111-V device structures [2], [4], [7].…”

“…Using the model described in [5], a simplified metal strip test structure, assuming a biaxial metallization stress for all contacts of lo7 Nm-', was simulated by means of a finite element method stress analysis. The results are shown in Fig.…”

“…The device coordinate system is chosen such that the z axis is in the [ O i 11 direction, the y axis in the If the metal strips are assumed to be deposited on the (100) GaAs surface lengthways along the y axis, and if the source-todrain direction is along the z axis, the stress-strain field exerted by the metal on the substrate varies only in the 2 -2 plane. The crystal displacements U and w in the 2 and z directions, respectively, are obtained by the 2-D finite element method [ 5 ] . The stress exerted on the GaAs is modeled by assuming that the forces at the metal edges parallel to the z axis point toward the metal strip [ 151, causing the GaAs regions below the metal to be under compression.…”