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We evaluated the band offsets at the SiO2/β-Ga2O3 (010) interface by X-ray photoelectron spectroscopy (XPS). Plasma chemical vapor deposition with a liquid tetraethyl orthosilicate source for SiO2 was used to prepare an SiO2(40 nm)/Ga2O3 sample and an SiO2(3 nm)/Ga2O3 sample for XPS analyses. The bandgap of SiO2 was determined to be 8.7 ± 0.2 eV. A large conduction band offset of 3.1 ± 0.2 eV and a corresponding valence band offset of 1.0 ± 0.2 eV were determined for the SiO2/Ga2O3 interface. These results suggest that an SiO2 gate insulator is favorable for Ga2O3 field effect transistors operating under high temperatures.

N-face AlGaN (cap)/GaN (channel)/AlGaN (barrier)/GaN (buffer) high electron mobility transistors (HEMTs) provide a simple solution for strong confinement of the two-dimensional electron gas (2DEG) from the back since carriers are induced on top of the AlGaN barrier. To reduce the adverse effects of random alloy scattering from the AlGaN barrier, an N-face GaN-spacer HEMT was designed with an epi-structure consisting of an AlGaN (cap)/GaN (channel)/AlN/GaN (spacer)/AlGaN (barrier)/GaN (buffer). The 2DEG is confined at the GaN (channel)/AlN interface, while the GaN-spacer layer separates the 2DEG from the AlGaN barrier. The large polarization-induced electric field of the AlN and its large conduction band discontinuity with GaN provide strong confinement for good pinch-off and low output conductance under high electric fields. Up to 20% improvement in electron mobility was measured in these devices. The devices showed a current gain cut-off frequency (f t ) of 24 GHz and power gain cut-off frequency (f max ) of 44 GHz for a nominal gate length of 0.7 μm. An output power density of 4.5 W/mm was measured at 4 GHz, with 34% power added efficiency and a gain of 10.3 dB at a drain bias of 40 V.

N-polar (000) and Ga-polar (0001) GaN templates were patterned with holographic lithography and etched to form nanopillars (NP) and nanostripes (NS) arrays, upon which InGaN and GaN was subsequently regrown by metal organic chemical vapor deposition (MOCVD). A wide range of growth conditions were explored to determine the structural impact of the growth conditions. The results were evaluated by scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was observed that the initial polarity of the GaN template used to form the pillar or stripe nanostructures had a significant impact upon the results of the regrowth. The Ga-polar NP exhibited (0001), {010}, and {011} facets, while the N-polar NP exhibited (000) and {010} facets. The NS samples also exhibited different facet formation depending upon the initial polarity. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)