We describe a proposed cascode GaN device configuration that allows stable operation during zero voltage switching (ZVS) turn-on transition and suppresses non-ZVS losses. We verified that application of our proposed device to an LLC resonant converter resulted in stable operation. In our device configuration, a GaN high-electron-mobility transistor (HEMT) gate is directly driven by a commercial Si MOSFET driver via a charge pump circuit. This allows the slew rate (dv/dt) to be controlled by an external gate resistance. In addition, we demonstrate that the 650-V normally-on GaN HEMT used in our proposed device configuration has highly reliable characteristics. The predicted lifetime for a 0.1% failure rate under actual bias conditions (Vds = 500 V at (150°C)) exceeds 1000 years (8.76 $\times $ 10 6 )hr).

Epitaxial growth of GaN films at a low temperature of 800 °C was studied in radical-enhanced metal-organic chemical vapor deposition, focusing on the discharge region of the plasma of a mixture of N2 and H2 gases. The effect of plasma confinement on the growth is crucial for realizing high crystal quality of grown GaN films, owing to the suppression of plasma-induced damage and decomposition of gallium precursors in the gaseous phase. By confined plasma in the discharge region using the plasma shield plates made of metal with multiple small holes effectively, GaN with a relatively flat surface was grown under conditions of higher V/III ratios. Epitaxial growth of GaN films was achieved by modifying the plate design and controlling the high V/III ratio using both the plasma-excitation power and the Ga precursor flow rate.

Nitrogen atoms are versatile for nitridation applications and do not lead to plasma-induced damage. Large-sized wafer processing demands a uniform supply of nitrogen atoms produced in a high-density very-high-frequency excited plasma of N2 without ammonia. The confinement of plasma through the use of a plasma shield plate (PSP) allows the samples to be separated in a downstream chamber. Generation and transport of N atoms were computationally simulated, and the PSP designs were implemented by PSP parameterization. The supply of high-density N radicals to the sample stage was optimally designed with sufficiently small holes and thin PSP to satisfy an aspect ratio of thickness-to-hole-diameter of less than 2.5.

Defect analysis of GaN films in high electron mobility transistor (HEMT) structures by cross-sectional cathodoluminescence (X-CL) is demonstrated as a useful technique for improving the current collapse of GaN-HEMT devices, and the relationship between crystal quality and device characteristics is also investigated. The crystal quality of intrinsic-GaN (i-GaN) and carbon-doped GaN produced clearly different peak intensities of blue luminescence (BL), yellow luminescence (YL), and band-edge emission (BE), which is independently detected by X-CL. Current collapse in GaN-HEMT devices is found to be determined by the BL/BE and YL/BE ratios at the top of the i-GaN layer, which is close to the channel. Moreover, the i-GaN thickness required in order to minimize the BL/BE and YL/BE ratios and the thickness dependency of GaN for minimizing the BL/BE and YL/BE ratios depending on the growth conditions can be evaluated by X-CL. However, there is no correlation between current collapse in GaN-HEMT devices and the YL/BE ratio by conventional photoluminescence because HEMT devices consist of multiple GaN layers and the YL signal is detected from the carbon-doped GaN layer. Thus, the X-CL analysis method is a useful technique for device design in order to suppress current collapse.

We investigated unintentionally doped nonpolar a- and m-plane GaN layers grown by metalorganic vapor phase epitaxy under several sets of conditions on freestanding a- and m-plane GaN substrates. Oxygen contamination in a-plane GaN is greatly reduced by increasing the V/III ratio during growth. As a result, a high-resistivity GaN buffer layer for an AlGaN/GaN heterostructure field-effect transistor was realized.

We measured drain bias stress effects and current collapse in AlGaN/GaN heterostructure field-effect transistors (HFETs) on a-plane and c-plane GaN substrates. An a-plane AlGaN/GaN HFET (a-HFET) shows small current collapse with a threshold voltage (V-th=-1.8 V). On the other hand, a c-plane HFET (c-HFET) with the same barrier thickness (20 nm) shows a small current collapse, although V-th was negatively large (V-th=-4 V). Current collapse in a-HFET was not large compared with that in conventional c-HFET on GaN. A c-HFET on sapphire was also measured. The current collapses in HFETs on sapphire were larger than that in HFETs on a GaN substrate. The current collapses in the thin-barrier c-HFET (Vth=-1.8 V) were particularly large. Therefore, an a-plane device is promising for a small or positive Vth with small current collapse. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report on the electrical properties of AlInN/GaInN heterostructures fabricated with InN molar fractions of 0 to 0.6 in the GaInN layer. High-density two-dimensional electron gases are formed near the interfaces of AlInN/AlN/GaInN at InN molar fractions of 0.3 and 0.6. The Al0.82In0.18N/AlN/Ga0.4In0.6N/GaN heterostructure field-effect transistors exhibited static characteristics. The maximum drain-source current reached a value of 0.26 A/mm. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report on the electrical properties of AlGaN/GaInN heterostructures fabricated with various InN molar fractions from 0 to 0.60 in GaInN on a GaN template. The sheet carrier density of the AlGaN/GaInN heterostructure monotonically increased with increasing InN molar fraction in GaInN, reaching 5.0 × 1013 cm−2 at an InN molar fraction of 0.60. The Al0.30Ga0.70N/Ga0.40In0.60N heterostructure exhibited static field‐effect transistor (FET) characteristics.

We found different behaviors of misfit dislocations in a Ga0.83In0.17N single layer and in Ga0.83In0.17N/Ga0.93In0.07N superlattices, both on GaN substrates. In the case of the single layer, misfit dislocations were formed at the GaInN/GaN interfaces and extended through the GaInN layer to the surface. In contrast, the misfit dislocations in the superlattices are bent laterally at interfaces between the Ga0.8.3In0.17N and Ga0.93In0.07N layers. In addition, most of the dislocations do not reach the surface owing to the formation of dislocation loops. As a result, the dislocation density at the surface of the GaInN superlattice sample was 5 x 10(7) cm(-2). (C) 2011 The Japan Society of Applied Physics

We optimized the initial GaN growths of nonpolar mand a-plane GaN grown on liquid phase epitaxy (LPE-) GaN substrates. Such nonpolar LPE-GaN substrates were produced by cutting slices from c-plane LPE-GaN bulk single crystals grown by the Na-flux method. We investigated the qualities of epitaxially grown GaN layers on the LPE-GaN substrates, with different polishing methods, which are chemical mechanical polishing and plasma dry etch polishing. We found that the crystalline quality of epitaxial GaN on m-plane GaN is very sensitive to the surface polishing even the minute unevenness in GaN substrate. Our experiments also indicated that a low initial growth rate was necessary to obtain high-crystallinequality epitaxial m-plane GaN. In contrast, highcrystalline-quality a-plane GaN was obtained even with a high initial growth rate, indicating that the crystalline quality of a-plane GaN is not sensitive to surface roughness. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report on the fabrication of GaInN-based solar cells using GaInN/GaInN superlattices as active layers and also as underlying layers beneath the active layers. We obtained pit-free surfaces, even with a high InN molar fraction, using the superlattices. As a result, the maximum external and internal quantum efficiencies reached 60%, and 88%, respectively. The open-circuit voltage of the soalr cells was 1.77 V, the short-circuit current density was 3.08 mA/cm(2), and the fill factor was 70.3%. A conversion efficiency of 2.46% was achieved at room temperature under simulared 1.5 sun x AM1.5G illumination using a solar simulator. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

High-performance GaInN-based solar cells with high open-circuit voltage, high short-circuit current density, and good fill factor have been obtained using a combination of two different GaInN superlattice structures. The GaInN barrier thicknesses (3 and 0.6 nm) in both superlattice structures were optimized, resulting in a thick GaInN-based active layer with a low pit density in the device. The conversion efficiency is approximately 2.5% under a solar simulator of air mass 1.5G and an irradiation intensity of 155 mW/cm2.

We fabricated and characterized AlGaN/GaN heterostructure growth by MOVPE on vicinal m-plane free-standing GaN substrates prepared by the Na flux method. The miscut angle in the LPE-GaN substrate has a great influence on the surface morphology and crystalline quality of epitaxial GaN and AlGaN films. In particular, AlGaN/GaN on a 48 miscut-angle LPE-GaN substrate shows large step bunching, small-miscut-angle LPE GaN substrate is essential for fabricating on FET structure with a flat surface. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We fabricated and characterized nonpolar a-plane AlGaN/GaN heterostructure field-effect transistors (HFETs) grown on an a-plane freestanding GaN substrate. By optimizing the growth conditions, the unintentionally doped oxygen concentration was much reduced in the a-plane GaN buffer layer. As a result, the low leakage current in the buffer layer was realized without doping of deep acceptors, such as Fe and C, by which an impurity-contamination-free channel layer can be successfully grown. A maximum drain current of 220 mA/mm at a gate source voltage of +3.0 V, an on resistance of 10.4 mΩ·cm2, and a threshold voltage of -1.6 V were realized.