Total-dose gamma radiation effects on static, high-frequency, and pulsed current-voltage characteristics of silicon nitride passivated AlGaN/GaN HEMTs were investigated. Passivated AlGaN/GaN HEMTs were exposed to a maximum total dose of 600 Mrad using a Co-60 source under nitrogen ambient. The magnitude of the changes in the DC characteristics exhibited a monotonic increase with increasing radiation dose. At 600 Mrad dose, -0.1 V shift in threshold voltage and 3% increase in maximum transconductance was observed. High-frequency small-signal characteristics did not exhibit a significant change. Pulsed current levels increased in relation to the shift in threshold voltage, indicating no radiation damage related degradation in large signal transient behavior. The mobility, sheet carrier density, contact resistance, and sheet resistance of the sample did not exhibit measurable changes. The limited response of the devices to the high radiation dose indicates that the AlGaN/GaN HEMTs can have very high radiation hardness. The observed effects are consistent with radiation-induced trap creation. (C) 2003 Elsevier Ltd. All rights reserved.