Improved DC performance of AlGaN/GaN high electron mobility transistors (HEMTs) have been demonstrated using reactive-sputtered hafnium oxide (HfO2) thin film as the surface passivation layer. Hall data indicate a significant increase in the product of sheet carrier concentration (n(s)) and electron mobility (mu(n)) in the HfO2-passivated HEMTs, compared to the unpassivated HEMTs. This improvement in electron carrier characteristics gives rise to a 22% higher I-Dmax and an 18% higher g(mmax) in HEMTs with HfO2 passivation relative to the unpassivated devices. On the other hand, I-gleak of the HEMTs decreases by nearly one order of magnitude when HfO2 passivation is applied. In addition, drain current is measured in the subthreshold regime. Compared to the unpassivated HEMTs, HfO2-passivated HEMTs exhibit a much smaller off-state I-D, indicating better turn-off characteristics. (c) 2006 Elsevier B.V. All rights reserved.