Unpassivated AlGaN/GaN high-electron-mobility transistors show significant gate lag effects due to the presence of surface states in the region between the gate and drain contact. Low-temperature (100 °C) layers of MgO or Sc2O3 deposited by plasma-assisted molecular-beam epitaxy are shown to effectively mitigate the collapse in drain current through passivation of the surface traps. These dielectrics may have advantages over the more conventional SiNX passivation in terms of long-term device stability.

We demonstrated that Sc2O3 thin films deposited by plasma-assisted molecular-beam epitaxy can be used simultaneously as a gate oxide and as a surface passivation layer on AlGaN/GaN high electron mobility transistors (HEMTs). The maximum drain source current, IDS, reaches a value of over 0.8 A/mm and is ∼40% higher on Sc2O3/AlGaN/GaN transistors relative to conventional HEMTs fabricated on the same wafer. The metal–oxide–semiconductor HEMTs (MOS–HEMTs) threshold voltage is in good agreement with the theoretical value, indicating that Sc2O3 retains a low surface state density on the AlGaN/GaN structures and effectively eliminates the collapse in drain current seen in unpassivated devices. The MOS-HEMTs can be modulated to +6 V of gate voltage. In particular, Sc2O3 is a very promising candidate as a gate dielectric and surface passivant because it is more stable on GaN than is MgO.

Articles you may be interested inDepletion-mode Ga2O3 metal-oxide-semiconductor field-effect transistors on β-Ga2O3 (010) substrates and temperature dependence of their device characteristics Appl. Phys. Lett. 103, 123511 (2013); 10.1063/1.4821858 GaN metal-oxide-semiconductor field-effect transistor inversion channel mobility modeling GaN metal-oxide-semiconductor high-electron-mobility-transistor with atomic layer deposited Al 2 O 3 as gate dielectric Appl. Phys. Lett. 86, 063501 (2005); 10.1063/1.1861122 Delta-doped AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with high breakdown voltages Appl.We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor ͑MOSFET͒ utilizing Si ϩ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ϳ14 V, corresponding to a breakdown field strength of 1.75 MV cm Ϫ1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage Ͼ15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 S mm Ϫ1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs.

Gate-controlled n+p metal–oxide–semiconductor diodes were fabricated in p-GaN using MgO as a gate dielectric and Si+ implantation to create the n+ regions. This structure overcomes the low minority carrier generation rate in GaN and allowed observation of clear inversion behavior in the dark at room temperature. By contrast, diodes without the n+ regions to act as an external source of minority carriers did not show inversion even at measurement temperatures of 300 °C. The gated diodes showed the expected shape of the current–voltage characteristics, with clear regions corresponding to depletion and inversion under the gate. The MgO was deposited prior to the Si implantation and was stable during the activation annealing for the Si-implanted n+ regions.

UV-ozone cleaning prior to metal deposition of either e-beam Pt contacts or sputtered W contacts on n-type single-crystal ZnO is found to significantly improve their rectifying characteristics. Pt contacts deposited directly on the as-received ZnO surface are Ohmic but show rectifying behavior with ozone cleaning. The Schottky barrier height of these Pt contacts was 0.70eV, with ideality factor of 1.5 and a saturation current density of 6.2×10−6Acm−2. In contrast, the as-deposited W contacts are Ohmic, independent of the use of ozone cleaning. Postdeposition annealing at 700°C produces rectifying behavior with Schottky barrier heights of 0.45eV for control samples and 0.49eV for those cleaned with ozone exposure. The improvement in rectifying properties of both the Pt and W contacts is related to removal of surface carbon contamination from the ZnO.