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The degradation mechanisms of 0.1 mum AlSb/InAs HEMTs subjected to elevated-temperature lifetesting at three temperatures in N2 atmosphere were investigated. Device degradation exhibits the increase of non-pinch-off drain current (IDS), the decrease of transconductance (gm) and the gate current (IG) increase. The IG increase was found to correlate with material degradation on the gate-recess and Al0.7Ga0.3Sb-mesa-floor surfaces. Higher oxygen content was detected on these surfaces, indicating that they were modified by oxidation, which resulted in the IG increase. Despite the degradation observed in 0.1 mum AlSb/InAs HEMTs, the three-temperature lifetesting shows that the activation energy (Ea) is approximately 1.5 eV and demonstrates a median time to failure (MTF)of 2times106 hours at Tjunction of 85degC. This reliability result is essential for successful insertion of AlSb/InAs HEMTs into systems with ultralow-power requirements. Moreover, ohmic-metal lateral diffusion of Pd and Au elements was observed. To avoid potential ohmic-metal-lateral-diffusion induced device failure, lifetesting temperatures were kept below 190degC in this investigation.