The effect of layer structure (GaN versus AlGaN cap) and cleaning procedure prior to Sc2O3 or MgO deposition at 100 degreesC were examined for their effects on the long-term bias-stress stability of AlGaN/GaN high electron mobility transistors (HEMTs). Surface cleaning by itself was not sufficient to prevent current collapse in the devices. The forward and reverse gate leakage currents were decreased under most conditions upon deposition of the oxide passivation layers. After approximate to13 h of bias-stressing, the MgO-passivated HEMTs retain greater than or equal to 90% their initial drain-source current. The Sc2O3-passivated devices retained similar to80% recovery of the current under the same conditions. (C) 2002 Published by Elsevier Science Ltd.