We review recent progress in obtaining low interface state densities on GaN and reducing current collapse with dielectrics on AlGaN/GaN high electron mobility transistors (HEMTs). New oxides of scandium and magnesium have shown promise for surface passivation on HEMTs however the lattice mismatches of -6.5% for MgO and +9.1% for Sc2O3 have led to efforts to find lower lattice mismatch oxides to increase oxide/nitride interfacial stability. By adding calcium to MgO, a crystalline film of MgCaO can be produced that has a closer lattice match to GaN. Stability of the dielectric films was determined for environmental and thermal processes and a 5 nm cap of Sc2O3 was found to increase the stability of the MgCaO over that of MgO and produce a 15% increase in carrier concentration over the non-passivated samples. This increase in sheet carrier density was maintained for several weeks at temperatures of 200 degrees C. (c) 2006 Elsevier Ltd. All rights reserved.