Yoshiharu Anda

99.3% Efficiency of three-phase inverter for motor drive using GaN-based Gate Injection Transistors

In this paper, we present a successful operation of Gallium Nitride(GaN)-based three-phase inverter with high efficiency of 99.3% for driving motor at 900W under the carrier frequency of 6kHz. This efficiency well exceeds the value by IGBT (Insulated Gate Bipolar Transistor). This demonstrates that GaN has a great potential for power switching application competing with SiC. Fully reduced on-state resistance in a new normally-off GaN transistor called Gate Injection Transistor (GIT) greatly helps to increase the efficiency. In addition, use of the bidirectional operation of the lateral and compact GITs with synchronous gate driving, the inverter is operated free from fly-wheel diodes which have been connected in parallel with IGBTs in a conventional inverter system.

Super self-aligned GaAs RF switch IC with 0.25 dB extremely low insertion loss for mobile communication systems

An extremely low loss switch IC has been implemented by using a 0.15 /spl mu/m-gate super self-aligned FET with reduced drain/source area. Both off-state-capacitance and the specific on-resistance of the implemented FET have been dramatically reduced by the novel device structure. The experimentally fabricated switch IC showed the low insertion loss of 0.25 dB at an added power of 35 dBm at a frequency of 0.9 GHz, which is the lowest value ever reported.

Blocking-voltage boosting technology for GaN transistors by widening depletion layer in Si substrates

We propose a novel technique to boost the blocking voltage of AlGaN/GaN hetero junction field effect transistors (HFETs) by widening a depletion layer in highly resistive Si substrate. The blocking-voltage boosting (BVB) technology utilizes ion implantation at the peripheral area of the chip as channel stoppers to terminate the leakage current from the interfacial inversion layers at AlN/Si. A depletion layer is widened in the substrate by the help of the channel stopper, which increases the blocking voltage of the HFET. The off-state breakdown voltage of the HFETs is increased up to 1340V by the BVB technology from 760V without the channel stoppers for the epitaxial GaN as thin as 1.4µm on Si. This technology greatly helps to increase the blocking voltage even for thin epitaxial GaN on Si, which leads to further reduction of the fabrication cost.

Recent Advances in GaN Power Switching Devices

Recent advances in GaN power switching devices are reviewed. A new normall-off GaN transistor called Gate Injection Transistor (GIT) increases drain current by conductivity modulation. The GIT is fabricated on cost-effective Si substrates by novel MOCVD technology enabling crack-free and smooth surfaces over 6-inch wafer. These technologies with thermally stable device isolation by Fe ion implantation are applied for a monolithic inverter IC. This is the world fist demonstration of a GaN inverter IC for motor drive, which reduces the total operating loss by 42% from that by the IGBT-based inverter. These technologies are indispensable for wide-spread use of GaN power switching transistors in the future.

GaN Gate Injection Transistor with integrated Si Schottky barrier diode for highly efficient DC-DC converters

In this paper, we present a novel GaN-based normally-off transistor with an integrated Si Schottky barrier diode (SBD) for low voltage DC-DC converters. The integrated SBD is formed by the Si substrate for the epitaxial growth of AlGaN/GaN hetero-structure, which is connected to the normally-off GaN Gate Injection Transistor (GIT) over it with via-holes. The diode can flow the reverse current in the conversion operation with lower forward voltage than that of the lateral GaN transistor enabling lower operating loss. A DC-DC converter from 12V down to 1.3V using the integrated devices with the reduced gate length down to 0.5μm exhibits a high peak efficiency of 89% at 2MHz demonstrating the promising potential of GaN devices for the application.

A Novel Thin Concentrator Photovoltaic With Microsolar Cells Directly Attached to a Lens Array

We propose a novel concept of thin and compact CPV modules in which submillimeter solar cells are directly attached to lens arrays without secondary optics or an extra heat sink. With this small cell size, the optical path length of the module can be brought down to one-twentieth that of conventional CPV modules. To achieve precise alignment of the microsolar cells at the lens focal points, we have developed a fluidic self-assembly technique that utilizes surface tension. This novel CPV module with triple junction microsolar cells demonstrated an efficiency of 34.7% under sunlight in the particular measured condition.

High-Voltage Isolation Technique Using Fe Ion Implantation for Monolithic Integration of AlGaN/GaN Transistors

Ion implantation technique can be applied for planar isolation of AlGaN/GaN heterojunction field-effect transistors (HFETs), which enables high-density integration of the power switching transistors. So far, the reported isolation using ion implantation for GaN devices has never maintained high isolation voltages after high-temperature processing over 800 °C which is commonly used for the fabrication. In this paper, we present detailed analysis and mechanism of thermally stable isolation of GaN devices by Fe ion implantation keeping high breakdown voltage between the devices after high-temperature annealing. Ion species forming deep levels at atomic sites in GaN are examined by using first-principle calculation prior to the experiments. The calculation indicates that the Fe ions stay at Ga sites with deep levels in GaN. The following experiments using various ion species well agree with the aforementioned predictions, where implanted regions by other ions than Fe exhibit reduction of the resistivity after high-temperature annealing to recover the processing damage by the ion implantation. As a result, it is experimentally found that Fe is the only choice to serve high resistivity after the annealing. The Fe ion implantation enables high breakdown voltage of 900 V after the annealing at 1200 °C. This technique is indispensable to enable monolithic integration of the lateral AlGaN/GaN HFETs for high-voltage power switching systems.

Comprehensive study on initial thermal oxidation of GaN(0001) surface and subsequent oxide growth in dry oxygen ambient

Initial oxidation of gallium nitride (GaN) (0001) epilayers and subsequent growth of thermal oxides in dry oxygen ambient were investigated by means of x-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy, and x-ray diffraction measurements. It was found that initial oxide formation tends to saturate at temperatures below 800 °C, whereas the selective growth of small oxide grains proceeds at dislocations in the epilayers, followed by noticeable grain growth, leading to a rough surface morphology at higher oxidation temperatures. This indicates that oxide growth and its morphology are crucially dependent on the defect density in the GaN epilayers. Structural characterizations also reveal that polycrystalline α- and β-phase Ga2O3 grains in an epitaxial relation with the GaN substrate are formed from the initial stage of the oxide growth. We propose a comprehensive model for GaN oxidation mediated by nitrogen removal and mass transport and discuss the model on the basis of ex...

A low-impedance coplanar waveguide using an SrTiO/sub 3/ thin film for GaAs power MMIC's

A novel structure for coplanar-waveguide transmission lines with low impedance and low loss is demonstrated in this paper. The new structure simply has a high dielectric SrTiO/sub 3/ thin film underneath the coplanar conductors. Due to the high dielectric constant of SrTiO/sub 3/, the coplanar line exhibited characteristic impedance as low as 18 /spl Omega/ with a slot width of 5 /spl mu/m and the center conductor width of 50 /spl mu/m, while a conventional coplanar line on GaAs showed only 30 /spl Omega/ with the same configuration. The newly developed coplanar structure is easily applicable for present GaAs monolithic-microwave integrated-circuit (MMIC) technology, especially for power MMICs and low-impedance devices.

High-efficiency thin and compact concentrator photovoltaics using micro-solar cells with via-holes sandwiched between thin lens-array and circuit board

We have developed a compact concentrator photovoltaic (CPV) module that comprises micro-solar cells with an area of ≈0.6 × 0.6 mm2 sandwiched between a 20-mm-thick lens array and a 1-mm-thick circuit board with no air gap. To establish electrical connections between the circuit board and the micro-solar cells, we developed a micro-solar cell with positive and negative electrodes on the lower face of the cell. In this study, we demonstrated the photovoltaic performance of the micro-solar cell closely approaches that of the standard solar cell measuring ≈5 × 5 mm2 commonly used in conventional CPVs under concentrated illumination. Our study showed that the negative effect on PV performance of perimeter carrier recombination in the micro-solar cell was insignificant under concentrated illumination. Finally, we assembled our micro-solar cells into a CPV module and achieved the module energy conversion efficiency of 34.7% under outdoor solar illumination.