W e have studied the degradation mechanism of AI- Cross+ectional transmission electron microscopy (TEM) images from the deteriorated devices reveal the existence of a damaged recess s u f u c e region at the drain side o f t h e device In this dumaged region. a significunt amount of oxygen is detected by energy dispersive xqay spectroscopy (EDX) ana/ysis. The dumuged recess region leads to a reducedzarrier density that results in decreased Imax

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. CitationHisaka, Takayuki et al. "Corrosion-induced degradation of GaAs PHEMTs under operation in high humidity conditions."

GaAs Pseudomorphic High-Electron Mobility Transistors (PHEMTs) are widely used in RF power applications. Since these devices typically operate at high power levels and under high voltage biasing, their electrical reliability is of serious concern. Previous studies have identified several distinct degradation phenomena in these devices, but a complete picture has yet to be formed.In this study, we have carried out a comprehensive study of the mechanisms of electrical degradation on a set of experimental RF power GaAs PHEMTs (non-commercial devices provided by our sponsor, Mitsubishi Electric). A wide variety of electrical stressing experiments employing different conditions (varying temperature, bias, environment) were performed on these devices in order to monitor their degradation with stressing.Our general observations showed several forms of degradation, the most concerning being an increase in the drain resistance RD and a reduction in maximum drain current Ima. Contrary to what is often claimed in the literature, our experiments indicated that these forms of degradation were not driven by impact-ionization or hot-electron effects. Instead, we found the degradation to be strongly correlated with temperature, stressing environment, and drain-gate bias, which were all consistent with a corrosion mechanism. Via materials analysis we were able to confirm that the degradation of both RD and I,., were due to surface corrosion on the drain side of the device, albeit at different specific locations. The increase in RD was attributed to oxidation on the n+GaAs ledge, while the reduction in I, was due to oxidation on the AlGaAs surface, closer to the gate.A recoverable negative shift in the threshold voltage VT and a permanent decrease in Rs were also observed during electrical stressing. The shift in VT was attributed to field-assisted tunneling of electrons out of traps under the gate, while the decrease in Rs was found to be consistent with recombination-induced annealing of defects on the source side of the device.

Si02Suc ri We have investigated drain degradation in a set of passivation Source G Drain experimental RF power GaAs PHEMTs. Drain degradation ohmics_ was observed in the form of an increase in RD and aca reduction in Imax in a variety of conditions. We found that tcGhstA both forms of degradation arise from surface corrosion that AIGaAs takes place on different locations on the drain and chantne dominate in different regimes of operation. Specifically, AIGaAs the increase in RD was prominent in the ON-state and was Gua found to be associated with corrosion on the drain n+GaAs ledge. The reduction in Imax, in contrast, was prominent in Figure 1: Schematic cross-section of GaAs PHEMT under study. Lg the OFF-state and was associated with corrosion on the 0.25 um,r BVDG,off=l2-l5 V, fT 40-50 GHz.

We have studied the electrical degradation of RF-power PHEMTs by means of in situ 2-D light-emission measurements. Electroluminescence originates in the recombination of holes that have been generated by impact ionization. The local light intensity, thus, maps the electric-field distribution at the drain side of the device. This allows us to probe the uniformity of electrical degradation due to electric-field-driven mechanisms. We find that electrical degradation proceeds in a highly nonuniform manner across the width of the device. In an initial phase, degradation takes place preferentially toward the center of the gate finger. In advanced stages of degradation, the edges of the device degrade at a preferential rate. We identify the origin of this behavior as a small systematic nonuniformity in the recess geometry that impacts the magnitude of the electric field on the drain of the device. Our research suggests that a close examination of the width distribution of electric field in RF-power PHEMTs (and FETs in general) is essential to enhance their long-term reliability.