Satoshi Tamura

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.

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.

A compact GaN-based DC-DC converter IC with high-speed gate drivers enabling high efficiencies

In this paper, we present a novel compact DC-DC converter IC in which normally-off GaN-GITs (Gate Injection Transistors) and gate drivers are integrated into one chip. The DC-DC converter IC can achieve higher efficiency and smaller chip size by reducing parasitic inductances between switching power devices and gate drivers. The gate driver, having a DCFL (Direct Coupled FET Logic) with a buffer amplifier which is consisted of a GaN-HFET (Hetero-junction FET) and GaN-GITs can operate with higher speed and lower power consumption. The fabricated DC-DC converter IC exhibits a peak efficiency as high as 86.6% at 2MHz for the 12V-1.8V conversion.

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.

High-speed switching and current-collapse-free operation by GaN gate injection transistors with thick GaN buffer on bulk GaN substrates

GaN-based normally-off Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction are fabricated on bulk GaN substrates. Thickness of insulating GaN buffer layer is increased up to 16 μm for the presented device from 5 μm for conventional GITs on Si. The thick buffer reduces the parasitic output capacitances, which enables fast turn-off switching. The thick buffer and the use of bulk GaN substrate help to improve the crystal quality of AlGaN/GaN so that the sheet resistance is reduced. Improved crystal quality together with reduced trap density successfully suppresses the current collapse up to 1 kV or higher of the applied drain voltage. The resultant RonQoss (Ron: on-state resistance, Qoss: output charge) as a figure-of-merit for high speed turn-off switching is reduced down to 940 mΩnC that is one third from that of GITs on Si. The resultant turn-off dVds/dt reaches as large as 285 V/ns that is twice higher than reported values by GITs on Si.

1.7 kV/1.0 mΩcm 2 normally-off vertical GaN transistor on GaN substrate with regrown p-GaN/AlGaN/GaN semipolar gate structure

A normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 1.7 kV is presented. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltages of 2.5 V and stable switching operations. Note that formation of carbon-doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. These results indicate that the demonstrated vertical GaN transistor is very promising for future high power switching applications.

GaN-based Gate Injection Transistors for power switching applications

GaN-based Gate Injection Transistors (GITs) with p-type gate over AlGaN/GaN heterojunction serve normally-off operations with low on-state resistances owing to the conductivity modulation by injection of holes. Established basic technologies on the GIT have shown promising features for switching applications. Further improvement of the performances would extend the applications and lead to the widespread use. In this paper, recent technologies on the GITs to improve the performances and extract the full potential are described. These include extension of the wafer diameter of Si up to 8 inch, InAlGaN quaternary alloy to reduce the series resistances, shortening the gate length to improve the device performances, integration of the gate driver and flip-chip assembly for faster switching.

A fully integrated GaN-based power IC including gate drivers for high-efficiency DC-DC Converters

In this paper, we present a state-of-the-art integrated GaN power IC capable of operating in a high frequency (MHz) regime. This realizes system size reduction, 60% maximum, of a power IC. The IC consists of two output power transistors (PT) and two gate drivers (GD). The key devices in the IC are normally-off gate injection transistors (GITs) for PT and GD and a normally-on hetero-junction field effect transistor (HFET) for GD. Novel local control of carrier concentration of an identical 2 dimensional electron gas (2DEG) at an AlGaN/GaN interface which made integration of the transistors with such a large threshold voltage difference possible is described. A specially developed post-passivation interconnection process giving low parasitic components is also described. The IC applied to a 12V-1.8V DC-DC converter shows high frequency switching operation well beyond the limit of Si pointing to future improvement in consumer electronics power supply systems.

A GaN-based surface-emitting laser with 45°-inclined mirror in horizontal cavity

A novel GaN-based surface-emitting laser was realized by utilizing total internal reflection (TIR) by an inclined mirror formed at one end of the horizontal cavity of an edge-emitting laser. The inclined mirror was fabricated by focused ion beam (FIB) etching. The mirror was inclined by 45° with respect to the surface normal. The guided light propagating along the horizontal-cavity is reflected at the mirror to the surface normal. We analyzed optical losses in the laser. To increase the external quantum efficiency, removal of an FIB-damaged layer and precise control of the mirror angle are important. Argon-milling was applied to the FIB-etched surface to remove the FIB-damaged layer which causes an optical loss. The fabricated device with the stripe width of 8 µm and the cavity length of 600 μm lased at 390 nm with a threshold current of 260 mA. Surface-emission was obtained with beam divergence angles of 24.0° and 6.2°, corresponding to perpendicular and parallel to the junction plane, respectively. The presented surface-emitting laser is suitable to form high-power GaN-based 2D laser arrays.

Vertical GaN-based power devices on bulk GaN substrates for future power switching systems

We propose a normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm2 and high off-state breakdown voltage of 1.7 kV. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltage of 2.5 V and stable switching operations. The employed p-type gate does not give any concern of the gate instability. Note that formation of carbon doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. We also propose a novel vertical GaN-based junction barrier Schottky (JBS) diode with trenched p-GaN region on a bulk GaN substrate. A specific differential on-resistance of the GaN JBS diode is 0.9 mΩ·cm2 while keeping high breakdown voltage of 1.6 kV. These results indicate that the demonstrated vertical GaN devices are very promising for future high power switching applications.