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The degradation mechanism of 0.1 μm InGaAs/InAlAs/InP HEMTs subjected to elevated temperature lifetest has been resolved with the techniques of scanning transmission microscope (STEM) and high-resolution energy-dispersive X-ray analysis (EDX). The results show that Schottky junction degradation is the dominant degradation mechanism, consisting of Ti inter-diffusion and In0.52Al0.48As Schottky barrier layer degradation. The degradation of the In0.52Al0.48As Schottky barrier exhibits the formation of TiAsx and indium-rich In0.52+xAl0.48As and/or indium depleted In0.52-xAl0.48As under elevated temperature lifetest. The Schottky junction degradation mechanism can be alleviated by using a new gate metal electrode technology (NGMET), which exhibits superior reliability performance to that of the Ti/Pt/Au gate metal electrode. Moreover, InP HEMT MMICs using NGMET exhibit comparable RF performance to that of InP HEMT MMICs with Ti/Pt/Au gate metal. The results achieved here demonstrate the further enhancement of 0.1 μm InP HEMT MMIC technology at Northrop Grumman Space Technology (NGST) using NGMET for military/space applications with high reliability performance requirement.