Thin (100 (A) over circle) layers of MgO or Sc2O3 grown by molecular beam epitaxy were investigated for surface passivation of AlGaN/GaN high electron mobility transistors. A variety of pre-treatments such as UV/O-3 or in situ heating were employed prior to the dielectric deposition. Under optimized conditions, the MgO produced an increase in drain-source current, threshold voltage and extrinsic transconductance, which is consistent with passivation of surface states in the AlGaN. The absence of hydrogen in the dielectrics makes them attractive candidates for long-term stable passivation of the HEMTs. (C) 2002 Published by Elsevier Science Ltd.