AbstractA series of constant drain current deep level transient and optical spectroscopies (CID‐DLTS/DLOS) is described, which allows characterization of deep levels with lateral spatial resolution in fully fabricated high electron mobility transistors (HEMTs). The techniques are used to examine the role of SiNx passivation in the access region of AlGaN/GaN HEMTs, in part to verify lateral resolution capabilities. Two distinct trap spectra were observed correlating to the under‐gate and access regions of the HEMT. Traps in the access region had energy levels of of EC‐0.43, EC‐1.3, EC‐2.3, and EC‐3.7 eV. The impact of each access region trap on the drain resistance before and after passivation was measured, with the total transient drain resistance change being reduced from 0.47 Ω‐mm to 0.11 Ω‐mm as a result of their partial passivation by SiNx, resulting in improved device performance. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)