This study investigates the device characteristics of InAs/AlSb HEMTs subjected to different periods of time storage in atmospheric ambiance after fabrication. Devices that have undergone 6 months of storage exhibit an increase of saturation drain current (I-DSS). increase of peak transconductance, decrease of gate leakage (I-G) and shifts of threshold voltage (Vth). The charge trapping effect was investigated by using a pulsed I-D-V-DS measurement, indicating that surface traps or defects were generated in the device that had undergone a 6-month storage. The decrease of I-G and shifts of Vth were found to correlate with material oxidization in the gate to the channel region, where an oxygen signal was detected by energy-dispersive analysis with X-ray (EDAX). Variances of gate capacitances (C-gs) extracted by the small-signal model were also used to justify the shifts of Vth Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.