Survivability of AlGaN/GaN HEMTs made with WNx- and Pt-based Gate Metal Stacks above 500 oC

Previous studies have demonstrated incompatibility of Ni/Au-based gates for operation AlGaN/GaN high-electron mobility transistors (HEMTs) at high temperatures [1]. At 400 oC and above, Ni diffused out of the HEMT interface and was replaced by Au – causing reliability issues. In this study, we explored options for blocking Ni diffusion by inserting a Pt barrier in between Ni and Au. We also developed a tungsten-nitride (WNx)-based gate metal stack (WNx/Pt/Au/Pt) that has shown 900 oC compatibility [2]. The DC characteristics of AlGaN/GaN HEMTs were tested at temperature (T) = 30-800 °C in vacuum (~ 1 mTorr) available in a Nextron Microprobe System. Raman spectroscopy was also collected during device measurement to ensure appropriate surface temperature. The devices were fabricated from identical epitaxy using optically defined metal gates (Lg ~ 0.7 µm). WNx/Pt/Au/Pt gated devices survived for the duration of the experiment (~ several hours) up to 700 °C, whereas the Ni/Pt/Au gated devices exhibited stable operation up to 500 oC. The measured device characteristics exhibited a reduction in on/off ratio with the increase in T, as expected from standard device physics. Cross-sectional scanning transmission electron microscopy (X-STEM) and energy-dispersive X-ray (EDX) analysis allowed us to observe the slight effectiveness of Pt in reducing the metal intermixing for the Ni/Pt/Au gate and high thermal stability for the WNx/Pt/Au/Pt gate.