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In this article, we present the results of on-wafer short-term (24 h) stress tests carried out on 0.25-μm AlGaN/GaN HEMTs. Devices on-wafer were submitted to 24-h dc tests, at various gate and drain voltage values corresponding to dissipated power densities PD up to 40 W/mm, with estimated channel temperature ≅ 375 °C. GEN1 devices adopted a Ni/Pt/Au gate metallization and conventional plasma-enhanced chemical vapor deposition (PE-CVD) SiN passivation; in GEN2 devices, a modified gate metallization and a two-layer SiN passivation were adopted. When tested at Pd > 25 W/mm, a substantial decrease of drain current ID and transconductance gm was measured in GEN1 HEMTs, without any significant shift of threshold voltage. Failure analysis revealed that Au and O interdiffusion took place from the sidewalls; Au gradually substituted Ni as a Schottky contact, while O, in the presence of high electric field, high temperature, and high current, promoted (Al)GaN oxidation and pitting. On the contrary, negligible degradation was found after high temperature storage of GEN1 devices without applied bias, up to 450 °C. In GEN2, process modification was effective in reducing the impact of this failure mechanism, resulting in only 5% gm decrease after 24 h at a junction temperature of 375 °C with PD = 38 W/mm. Results demonstrate the effectiveness of the adopted on-wafer screening methodology in identifying potentially dangerous failure mechanisms. [ABSTRACT FROM AUTHOR]

Current transient spectroscopy was used to measure the impact of neutron irradiation on output current-limiting charge traps in AlGaN/GaN high electron mobility transistors with time constants from 10 ms to 1800 s. We find that coupling between discrete traps was apparent, in contrast to the commonly employed assumption of independent trap (dis)charging, and increased after 14-MeV neutron irradiation of $2 \times 10^{13}\,\,\text {n/cm}^{2}$ and above. Irradiation to a high dose of as much as $7.8 \times 10^{14}\,\,\text {n/cm}^{2}$ , which is comparable to eight years exposure to a harsh radiation environment, such as the International Thermonuclear Experimental Reactor neutral-beam injector prototype, increased trapped charge density and reduced transient drain current to as little as 75% of its equilibrium value. These changes are consistent with displacement damage estimates based on the radiation transport calculations. [ABSTRACT FROM AUTHOR]

An experimental method to measure the gate metal temperature of GaN-based high electron mobility transistors is demonstrated. The technique is based on transient thermoreflectance measurements performed from the backside of the device. The thermoreflectance coefficient of the gate metal was calibrated by correlating the relative change of its optical reflectivity with the temperature change measured in the GaN layer using time-resolved Raman thermography during the device cooling transient. Simulated temperature transients were in good agreement with the experimental data. The main advantage of this new method is that it enables the direct assessment of gate metal temperature under device pulsed operation regardless of the device design. [ABSTRACT FROM PUBLISHER]