Electrical degradation mechanisms of RF power GaAs PHEMTs

Abstract

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.

Degradation mechanisms of GaAs PHEMTs in high humidity conditions

“…V th shifts after several types of accelerated tests have been investigated, and several mechanisms have been reported: gate metal sinking during high temperature stress [8,9], reversible V th change due to traps under gate via electric field stress [10,11], and V th shift by piezoelectric effect [12] due to stress change near the gate by hydrogen poisoning [13]. Gate sinking or other damage under the gate was not observed in our TEM images.…”

Degradation Uniformity of RF-Power GaAs PHEMTs Under Electrical Stress

“…The structure of this device has been described before [6]. Typical electrical characteristics of W g = 50-µm devices are f T ∼ 40−50 GHz, V T = −0.65 V, I max = 470 mA/mm, and BV DG,off ∼ 12−15 V [12].…”

Performance characterization of III–V power devices

“…The threshold voltage became more negative after large signal conditions had been applied. Under these large signal conditions a negative gate current was measured, which could be attributed to impact ionization [1,2] in the device. Impact ionization leads to significant generation of electron-hole pairs and these excess carriers contribute to the negative gate current.…”

“…The performance of high power amplifiers (HPAs) is continually being improved in terms of power density, efficiency and gain without any reduction in reliability requirements [1][2][3]. Efforts in this direction have become a major driving force of device evolution with a focus on device structure and material composition [4,5].…”