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Anomalous turn off (ATO) is observed for the GaN-based normally-off Gate injection Transistor (GIT) with a threshold voltage having smaller than 0.7 V. On the other hand, ATO is not observed for the GIT with a threshold voltage larger than 0.7 V. The commercialized GITs are free from the ATO, because their threshold voltages are controlled to be 1. 2 ±0. 3V, which is larger than 0.7 V. Device simulation study indicates that a donor-type hole trap is responsible for the ATO, because the internal electric field is large enough to induce the emission of the captured hole traps that are made when the device is turned off. It is simulated that the specified donor-type hole traps influence minor effects on the switching behavior.

The simple and high-accuracy equivalent circuit model for GaN-GIT bi-directional switch have been constructed by eliminating the gate resistance dependence from the capacitance parameters and employing the parallel gate resistance circuit with the diode. By the new methods, the calculated waveforms, switching losses and switching times agree well with the experimental ones, and the accuracy between the calculations and the experiments are more than 90%. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Status of the reliability study on silicon carbide (SiC) power MOS transistors is presented. The SiC transistors studied are diode-integrated MOSFETs (DioMOS) in which a highly doped n-type epitaxial channel layer formed underneath the gate oxide acts as a reverse diode and thus an external Schottky barrier diode can be eliminated. The novel MOS device can reduce the total area of SiC leading to potentially lower cost as well as the size of the packaging. After summarizing the issues on reliability of conventional SiC MOS transistors, the improvements by the newly proposed DioMOS with blocking voltage of 1200 V are presented. The I–V characteristic of the integrated reverse diode is free from the degradation which is typically observed in conventional pn-junction-based body diode in SiC MOS transistors. The improved quality of the MOS gate in the DioMOS results in very stable threshold voltage within its variation less than 0.1 V even after 2000 h of serious gate voltage stresses of + 25 V and − 10 V at 150 °C. High temperature reverse bias test (HTRB) shows very stable off-state and gate leakage current up to 2000 h under the drain voltage of 1200 V at 150 °C. These results indicate that the presented DioMOS can be applied to practical switching systems free from the reliability issues.

Trade-off between threshold voltage and specific on-resistance is successfully overcome in a diode-integrated SiC MOSFET by improving the design of n-type epitaxial channel layer and p-type body region. This new design features enhanced transconductance, hence low on-state resistance, while retaining high threshold voltage. Obtained specific on-resistance of the fabricated 1200V SiC DioMOS is among the lowest achieved for SiC MOSFETs including trench devices. The transconductance enhancement is also demonstrated to be effective in increasing the turn-on switching speed, thus contributing to higher efficiency in power switching systems with reduced conduction and switching losses.

Highly efficient three-phase to three-phase matrix converters using Gallium nitride (GaN) bidirectional switches with both high current and high breakdown voltage are demonstrated. The GaN switch with dual gates works as a bidirectional switch by a single device, while a conventional bidirectional switch consists of four devices by two Insulated Gate Bipolar Transistors (IGBTs) and two diodes. In addition, the GaN bidirectional switch is also free from the voltage offsets for the current conduction so that the GaN-based matrix converter enables small size and highly efficient AC/AC conversion. Improvement of the device performance including the introduction of the recessed gate enables the low on-state resistance with stable operation free from current collapse. The maximum drain current reaches 100 A together with the breakdown voltage of 1340 V. The fabricated three-phase to three-phase matrix converter exhibits the maximum conversion efficiency of 98% at 1 kW output power with the expectation that the maximum output power can reach 10 kW or more by the high current device.

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.

In this paper, high current and high voltage AlGaN/GaN metal-insulator-semiconductor (MIS) heterojunction field-effect transistors (HFETs) on Si are demonstrated. The devices exhibit a drain current of 20 A as well as a breakdown voltage of 730 V, serving normally-off operations. Stable interfacial characteristics free from the hysteresis in the transfer characteristics are enabled by the introduction of AlON gate insulator. A recessed gate structure formed by epitaxial regrowth of AlGaN over the grooved AlGaN/GaN heterojunction successfully reduces the on-state resistance and eliminates the processing damage on the surface of the grooved structure. Note that an oxygen annealing followed by the deposition of AlON shifts the threshold voltage V th to positive side. The resultant switching performance by the 20 A / 730 V AlGaN/GaN MIS-HFET is very fast with dV/dt of 78 V/ns and 169 V/ns for turn-on and turn-off transitions, respectively, indicating that the proposed MIS-HFETs are very promising for practical power switching applications.

We have demonstrated hysteresis-free recessed gate AlGaN/GaN metal-oxide-semiconductor heterojunction field-effect transistor (MOS-HFET) by implementing AIGN gate insulator and selective AlGaN regrowth technique. High thermal stability and excellent electrical properties of AIGN gate dielectrics will provide a large process window for further optimization of AlGaN/GaN MOS-HFET.

Reliability tests on 600V-rated GaN-based normally-off hybrid-drain-embedded Gate Injection Transistor (HD-GIT) are performed. High-temperature reverse-bias test on HD-GIT reveals that the lifetime is dependent on the leakage current before the reliability test. Acceleration factors for the temperature and reverse bias voltage are extracted. Based on the obtained results, devices are designed so that their lifetimes under Vbs=480V (80% de-rated) at 80°C are longer than 1000 years, which are long enough for most conventional power converter applications. Dynamic high-temperature operation life test is also performed, under which their DC characteristics are within their normal range up to 3600h. In addition, HD-GITs are within their normal range under the similar reliability tests as applied for Si-based power transistors. The physical mechanisms for the reliability of HD-GIT are discussed. We conclude that the OFF-state reliability is explained by the so-called percolation degradation model, and that the hole injection from the p-type GaN embedded in the drain plays an important role to suppress the degradation under hard-switching operation test.

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.

The radiated emission from the GaN-HEMT switching converter is investigated by two-dimensional electromagnetic-probe scanning. By the scanning, the single peak spectrum due to the ringing of the switching waveforms is obtained. When the supply voltage of the converter Vcc is varied, this peak frequency of the spectrum of the electromagnetic field is shifted. This peak shift is thought to be due to the change of the capacitance of the depletion region in the GaN-HEMTs. Therefore, we propose the equivalent circuit model for the radiated emission employing the resonant frequency between the output capacitance of the GaN-HEMTs and the parasitic inductance of the circuit wire Lline. The calculated results at Lline of 1.2 nH agree well with the experimental results. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 R on Q oss (R on : on-state resistance, Q oss : 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 dV ds /dt reaches as large as 285 V/ns that is twice higher than reported values by GITs on Si.

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.

The status and challenges in the development of GaN power devices are reviewed. At present, normally-off gate injection transistors (GITs) on Si are commercially available. The updated structure known as a hybrid-drain-embedded GIT provides superior reliability that contributes to the stable operation of compact power switching systems with high efficiency. The fabricated vertical GaN transistor on GaN as a future challenge demonstrates extremely low specific on-state resistance and high breakdown voltage. Metal-insulator-semiconductor-gate GaN transistor is also a technical challenge for faster switching, since it would give greater freedom of gate driving as a result of both high threshold voltage and widened gate voltage swing. Normally-off operation free from hysteresis in the current–voltage characteristics is confirmed in a recessed-gate AlGaN/GaN heterojunction field effect transistor using AlON as a gate insulator. Fast switching characteristics are experimentally confirmed for both of the newly developed GaN devices, indicating their great potential for practical use.

In this paper, we investigated the effects of the post-deposition annealing (PDA) on the threshold voltage (V th) of AlGaN/GaN MOS heterojunction field-effect transistors with atomic-layer-deposited Al2O3 using H2O vapor or oxygen plasma as the oxidant. By PDA, the V th shifts positively with the V th variations depending on the oxidants. The capacitance–voltage measurements reveal that the V th variation is attributed to the differences in the initial fixed charge density in the Al2O3 or/and the Al2O3/AlGaN for both oxidants.

This paper reviews the recent activities for normally-off GaN-based gate injection transistors (GITs) on Si substrates and their application to inverters. Epitaxial growth of the AlGaN/GaN heterostructures with good crystallinity over 200-mm Si substrates with eliminated bowing enables low-cost fabrication of GaN devices with high breakdown voltages. A novel normally-off GaN transistor called as GIT is proposed in which hole injection from the p-type AlGaN gate increases the drain current with low on-state resistance by conductivity modulation. The low on-state resistance in GaN-based devices greatly helps to increase the efficiency of power switching systems. A GaN-based three-phase inverter successfully drives a motor with high efficiency of 99.3% at a high output power of 1500 W. The presented GaN-based devices are expected to greatly help saving energy in the future as an indispensable power switching system.

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.

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.

We obtain an equivalent-circuit-model for GaN gate-insulated-transistor (GIT) bi-directional switches (BDSWs) which consists of one current source and three capacitances. Employing this model for the chopper circuit simulation, the gate resistance dependence is included into the input capacitance of CG1S2 and CG1S1 in order to improve the accuracy. By including this influence of the gate resistance, the calculated waveforms and the estimated switching losses agree well with the experimental ones with accuracy of over 90% when the circuit conditions are varied. This gate resistance dependence is considered to be caused by the hole injection at the gate region. By using our proposed model, this equivalent-circuit-model for GIT BDSWs can be treated with like a simple single-gate MOSFET model (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent progress in GaN-based power electron devices and advanced thermal management techniques for high performance GaN power devices are described. Recent advances in high performance GaN-based transistors are reviewed. The reduction of junction temperatures in low-pressure direct liquid cooling of GaN power devices for high-power and high-voltage applications are demonstrated using perfluorocarbon and perfluoroether as a working fluid, for the first time.

This paper reviews novel design of epitaxial structure for GaN-based power devices taking advantages of the material's unique polarization. This can be called as polarization engineering which enables low on-state resistances and high breakdown voltages. AlGaN/GaN superlattice and polarization-matched InAlGaN quaternary alloy capping layers effectively reduce the series resistance of AlGaN/GaN heterojunction field effect transistors (HFETs) by reducing the potential barriers above the heterojunction. Natural super junction (NSJ) model is proposed, which well explains the limitless increase of the breakdown voltages of GaN transistor by the extension of the gate-drain spacing. This model is applied to diodes with multi channels of AlGaN/GaN resulting in low on-state resistances and high breakdown voltages. The presented polarization engineering is very promising for future GaN power devices with the improved performances.

The impacts of inserting ultrathin oxides into insulator/AlGaN interfaces on their electrical properties were investigated to develop advanced AlGaN/GaN metal–oxide–semiconductor (MOS) gate stacks. For this purpose, the initial thermal oxidation of AlGaN surfaces in oxygen ambient was systematically studied by synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) and atomic force microscopy (AFM). Our physical characterizations revealed that, when compared with GaN surfaces, aluminum addition promotes the initial oxidation of AlGaN surfaces at temperatures of around 400 °C, followed by smaller grain growth above 850 °C. Electrical measurements of AlGaN/GaN MOS capacitors also showed that, although excessive oxidation treatment of AlGaN surfaces over around 700 °C has an adverse effect, interface passivation with the initial oxidation of the AlGaN surfaces at temperatures ranging from 400 to 500 °C was proven to be beneficial for fabricating high-quality AlGaN/GaN MOS gate stacks.

In this paper, the current–voltage (I–V) characteristics of a 600-V-class normally off GaN gate injection transistor (GIT) from 25 to 200 °C are analyzed, and it is revealed that the drain current of the GIT increases during high-temperature operation. It is found that the maximum drain current (I dmax) of the GIT is 86% higher than that of a conventional 600-V-class normally off GaN metal insulator semiconductor hetero-FET (MIS-HFET) at 150 °C, whereas the GIT obtains 56% I dmax even at 200 °C. Moreover, the mechanism of the drain current increase of the GIT is clarified by examining the relationship between the temperature dependence of the I–V characteristics of the GIT and the gate hole injection effect determined from the shift of the second transconductance (g m) peak of the g m–V g characteristic. From the above, the GIT is a promising device with enough drivability for future power switching applications even under high-temperature conditions.

Stacked gate dielectrics consisting of wide bandgap SiO2 insulators and thin aluminum oxynitride (AlON) interlayers were systematically investigated in order to improve the performance and reliability of AlGaN/GaN metal–oxide–semiconductor (MOS) devices. A significantly reduced gate leakage current compared with that in a single AlON layer was achieved with these structures, while maintaining the superior thermal stability and electrical properties of the oxynitride/AlGaN interface. Consequently, distinct advantages in terms of the reliability of the gate dielectrics, such as an improved immunity against electron injection and an increased dielectric breakdown field, were demonstrated for AlGaN/GaN MOS capacitors with optimized stacked structures having a 3.3-nm-thick AlON interlayer.

Alumina incorporating nitrogen (aluminum oxynitride; AlON) for immunity against charge injection was grown on a AlGaN/GaN substrate through the repeated atomic layer deposition (ALD) of AlN layers and in situ oxidation in ozone (O3) ambient under optimized conditions. The nitrogen distribution was uniform in the depth direction, the composition was controllable over a wide range (0.5–32%), and the thickness could be precisely controlled. Physical analysis based on synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) revealed that harmful intermixing at the insulator/AlGaN interface causing Ga out-diffusion in the gate stack was effectively suppressed by this method. AlON/AlGaN/GaN MOS capacitors were fabricated, and they had excellent electrical properties and immunity against electrical stressing as a result of the improved interface stability.

Reduction of thermal resistance in AlGaN/GaN heterojunction field-effect transistors (HFETs) is critical for further increase in their output power to be handled in these promising material systems. In this paper, we present a new technique to reduce it using polycrystalline AlN passivation deposited by dc sputtering as a surface heat spreader over AlGaN/GaN HFETs. The AlN is deposited by dc sputtering, and it is well oriented to the c-axis. The measured thermal resistances of AlGaN/GaN HFETs with AlN passivation are lower than those with SiN passivation, which well agree with the results of the thermal simulation. The most notable change of the characteristics by AlN passivation is that the drain current increases by 30%, and the on-state resistance is reduced by 66% by the passivation; moreover, the current collapse suppresses successfully. As a result, the radio frequency output power is higher than that with conventional SiN passivation and is increased by increasing the thickness of AlN passivation. Thus, reduction of the thermal resistance and higher output power by AlN passivation is experimentally achieved.

Nonpolar AlGaN/GaN metal-insulator-semiconductor-heterojunction field-effect transistors (MIS-HFETs) with a complete normally off operation have been demonstrated for the first time. To realize the normally off operation of the MIS-HFETs, a 2-nm-thick SiN (as an insulator) and GaN-based nonpolar epitaxial layers that are free from polarization charges are employed. We have found that a thicker nonpolar AlN buffer layer achieves a GaN layer with a narrower full width at half maximum of the X-ray rocking curve and higher electron mobility. The fabricated MIS gate structure on the AlN buffer layer successfully decreases the gate leakage current and enables a more positive gate bias of up to +4 V, which is advantageous to achieve a higher drain current. Moreover, the n-GaN capping layer between gate and ohmic electrodes helps to reduce parasitic resistance and suppress current collapse. The fabricated a-plane MIS-HFET exhibits a threshold voltage of + 1.3 V with a high drain current of 112 mA/mm. The presented MIS-HFETs will be desirable in next-generation radio-frequency and power switching application fields.

We have fabricated III-nitride metal-oxide field effect transistors (MOSFETs) using high- k HfO 2 as a gate oxide. Two types of MOSFETs were studied; GaN MOSFETs and AlGaN/GaN MOSFETs. In the case of GaN MOSFETs, the maximum transconductance of 45 mS/mm has been obtained. This is seven times larger than the best-reported value, to our knowledge, for the normally-off GaN MOSFETs with SiO 2 gate oxide. In order to improve the performance of the device, AlGaN/GaN MOSFETs in which high-quality AlGaN/GaN heterointerface is used as a channel have been fabricated. The maximum transconductance and drain current were as high as 160 mS/mm and 840 mA/mm, respectively.

In this paper, recent advances in GaN-based transistors for power switching and millimeter wave communications are reviewed. These two applications are emerging in addition to the widely developed power amplifiers at microwave frequencies mainly for cellular base stations. Reduction of the fabrication cost is strongly required for power switching GaN transistors, which is enabled by our epitaxial growth technology on large area Si substrates. Another requisite for the application is to achieve normally-off operation and our novel device structure called Gate Injection Transistors (GIT) enables it with low enough specific on-resistance (R on · A) and high drain current. Here, we also present the world highest breakdown voltage of 10400 V in AlGaN/GaN HFETs, extracting full advantage of the high breakdown strength of GaN. The used poly AlN passivation works as a surface heat spreader with its high thermal conductivity, which effectively relieves the channel temperature resulting in lower thermal resistances. The GaN device for future millimeter wave communication consists of MIS-type gate using so-called "in-situ" deposited SiN as a gate insulator, which exhibits high f max of 203 GHz and low noise figure of 1.4 dB at 28 GHz. The presented GaN transistors are promising for the two future emerging applications demonstrating high enough potential of the material systems.

We review our state-of-the-art GaN -based device technologies for power switching at low frequencies and for future millimeter-wave communication systems. These two applications are emerging in addition to the power amplifiers at microwave frequencies which have been already commercialized for cellular base stations. Technical issues of the power switching GaN device include lowering the fabrication cost, normally-off operation and further increase of the breakdown voltages extracting full potential of GaN -based materials. We establish flat and crack-free epitaxial growth of GaN on Si which can reduce the chip cost. Our novel device structure called Gate Injection Transistor (GIT) achieves normally-off operation with high enough drain current utilizing conductivity modulation. Here we also present the world highest breakdown voltage of 10400V in AlGaN / GaN HFETs. In this paper, we also present high frequency GaN -based devices for millimeter-wave applications. Short-gate MIS-HFETs using in-situ SiN as gate insulators achieve high fmax up to 203GHz. Successful integration of low-loss microstrip lines with via-holes onto sapphire enables compact 3-stage K -band amplifier MMIC of which the small-signal gain is as high as 22dB at 26GHz. The presented devices are promising for the two future emerging applications demonstrating high enough potential of GaN -based transistors.

In this paper, we demonstrate a novel method to integrate photonic crystals (PhCs) on GaN-based blue light-emitting diodes (LEDs) using a silicon substrate as a mold for forming the PhCs. This method starts with fabricating a 2D grooved Si substrate as that mold. Subsequently, GaN-based epitaxial layers are grown on the Si mold-substrate, which effectively reduces the dislocation density in GaN by enhanced lateral epitaxial growth. After the epitaxial layers are bonded onto a highly reflective substrate, the Si mold-substrate is removed. This substrate-transfer technique replicates PhC from the mold-substrate on the LED surface free from processing damages. The resultant LEDs with PhC have outperformed the LEDs without PhC in the optical output power by 80%, taking advantage of the enhanced light extraction by PhC.

We demonstrate polarization control of vertical-cavity surface-emitting lasers (VCSELs) to any desired directions by utilizing surface plasmon resonance at a metal nanohole array. A silver nanohole array is integrated on the p-type distributed Bragg reflector of an 850-nm AlGaAs-GaAs-based VCSEL. The metal nanohole array placed at rectangular lattices with orthogonally different two periods, one of which meets the resonaint condition, exhibits strong polarization preference in optical transmission. Integrated four VCSELs with different directions of the rectangular arrays of nanoholes demonstrate successful control of the polarization angles. The VCSELs are closely located onto a one chip so that the optical outputs are easily coupled to one optical fiber. The presented VCSEL array will open a new horizon of multi-input multi-output communication systems by using polarization multiplexing.

AlGaN/GaN MIS-HEMTs using HfO2 gate insulator with a large dielectric constant have been fabricated. HfO2 gate insulator was deposited by pulsed laser deposition at room temperature. It has been demonstrated that the HfO2 gate insulator is effective in suppressing the gate leakage current keeping a large transconductance. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lateral epitaxial growth (LEG) is a key technology to improve the lifetime of III-V nitride-based laser diodes (LDs) by reducing the dislocation density in the materials. To increase the area of low dislocation density, the lateral growth rate needs to be increased. In addition, suppression of the vertical growth is strongly desired to avoid unnecessarily thick growth, which would result in cracks in the epitaxial film. This paper reports the maskless LEG of GaN with extremely high lateral-to-vertical growth rate ratio using dimethylhydrazine as a nitrogen precursor. The lateral growth only occurs from the sidewalls of the etched mesa stripes without any dielectric masks. The lateral growth rate toward the $$ {{ }} $$ direction is extremely high, as high as 10 μm/h, while no vertical growth is observed on the top of unmasked mesa. The cross-sectional transmission electron microscopic image shows that the threading dislocations in the wing region extend only toward the lateral $$ {{ }} $$ direction. Note that almost smooth coalescence between the wing regions is confirmed by atomic force microscopy. X-ray diffraction measurements reveal that this maskless LEG drastically improves the crystallographic twist down to 97 arc-s, which is as comparably low as that of a free-standing GaN substrate. The presented maskless LEG is advantageous for optical device applications.

Electron backscatter diffraction (EBSD) technique has been applied to the strain distribution measurements for GaN based heterostructures for the first time. From the results for the simple AlGaN/GaN structures on GaN substrates, it was confirmed that this method has high spatial resolution of about 80nm and gives quantitatively reasonable strain values. The EBSD technique was then applied to the heterostructures grown on Si substrates to study the role of an AlN/GaN multilayer prior to the growth of a thick GaN layer. It was clarified that there exists a compressive strain in the GaN layer, especially near the interface to the AlN/GaN multilayer. This compressive strain will be relevant to the suppression of the crack generation in the thick GaN layer.

We have developed a normally-off GaN-based transistor using conductivity modulation, which we call a gate injection transistor (GIT). This new device principle utilizes hole-injection from the p-AlGaN to the AlGaN/GaN heterojunction, which simultaneously increases the electron density in the channel, resulting in a dramatic increase of the drain current owing to the conductivity modulation. The fabricated GIT exhibits a threshold voltage of 1.0 V with a maximum drain current of 200 mA/mm, in which a forward gate voltage of up to 6 V can be applied. The obtained specific ON-state resistance (RON . A) and the OFF-state breakdown voltage (BV ds) are 2.6 mOmega . cm2 and 800 V, respectively. The developed GIT is advantageous for power switching applications.

Current collapse at high drain voltage in a GaN-based transistor is successfully suppressed by the introduction of p-GaN region which is placed beside the drain of a Gate Injection Transistor (GIT). The additional p-GaN region enables hole injection which effectively releases trapped electrons at around drain region after the application of high drain voltages. The p-GaN region is electrically connected to the drain electrode so that this is named as Hybrid Drain-embedded GIT (HD-GIT). The fabricated HD-GITs are free from current collapse at 850 V of the drain voltage or over, which significantly helps to achieve stable system operations and is very promising for future switching power supply 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.

In this paper, we present recessed AlGaN/GaN heterojunction field-effect transistors (HFETs) with lattice-matched InAlGaN capping layers, which reduce both ohmic contact resistance and series resistance between the AlGaN and the capping layer. The lattice-matched alloy epitaxial layer with both In and Al high compositions are successfully grown by metal-organic chemical vapor deposition. The grown lattice-matched In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N capping layer has close total polarization and bandgap to those of the underlying Al/sub 0.26/Ga/sub 0.74/N layer. The balanced polarization eliminates the depletion of electrons at the In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N/Al/sub 0.26/Ga/sub 0.74/N interface, which can reduce the series resistance across it. It is also noted that the fabricated HFET exhibits very low ohmic contact resistance of 1.0/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ or less. Detailed analysis of the source resistance reveals that the series resistance at the In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N/Al/sub 0.26/Ga/sub 0.74/N interface is one fifth as low as the resistance at the conventional GaN/Al/sub 0.26/Ga/sub 0.74/N interface.

We have developed a high-power AlGaN/GaN HFET fabricated on 4-in conductive Si substrate with a source-via grounding (SVG) structure. The SVG structure enables efficient chip layout and high packing density by the vertical configuration. By establishing a high-quality epitaxial technology on a Si substrate and by significantly reducing the parasitic resistance, a very low specific on-state resistance of 1.9 m/spl Omega//spl middot/cm/sup 2/ is achieved. The breakdown voltage is as high as 350 V, which is attributed to the Si substrate acting as a backside field plate. Because of reduction of the parasitic inductance, very high level of current (2.0 kA/cm/sup 2/) transients, i.e., a turn-on time of 98 ps and a turn-off time of 96 ps, are successfully measured for the first time.

A vertical InGaN-based blue light emitting diode (LED) with a reflective metal electrode and thick gold plated base free from insulating sapphire is demonstrated. A 50 μm-thick gold plated layer on a highly reflective platinum p-side ohmic contact is formed on the InGaN-based LED epi-structure, followed by irradiation with high power ultra-violet pulsed laser from the backside of the original sapphire substrate. The laser processing, so called laser lift-off technique, enables the separation of sapphire by decomposition of interfacial GaN layer. No transparent electrode is necessary for the LED because the current can spread well through the highly conductive n-GaN layer so that uniform blue light is emitted through the n-GaN layer. The electroluminescence (EL) spectra show small multi peaks due to the resonance within the formed vertical cavity. In addition, the measured thermal resistance of the vertical LED is smaller than that of the conventional LED on poor heat-conductive sapphire. The vertical LED would increase the brightness together with smaller chip size and lower series resistance. This would be advantageous espacially for high power and highly efficient LEDs applicable to future solid state lighting. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A very thin 1 μm-thick Al 0.11 Ga 0.89 N film is successfully separated from a sapphire substrate by thermal decomposition of interfacial region using high power laser, so-called laser lift-off technique. The used layer structure prior to the separation is 1.0 μm Al 0.11 Ga 0.89 N/0.3 μm GaN on sapphire grown by metal organic chemical vapor deposition (MOCVD), and third harmonic Q-switched Nd:YAG laser ( λ =355 nm) is irradiated from the backside of the sapphire. Absorption of the laser light occurs only at the GaN interlayer and the decomposed thickness is limited up to the interlayer thickness. As a result, a crack-free thin AlGaN film is separated from sapphire with metal Ga underneath through the optimization of the process parameters. In contrast, removal of 2 μm-thick GaN single layer from sapphire is also tested, where peeling-off of the film with cracks is seen. In addition, slight red shift of the peak energy in the cathodoluminescence (CL) spectra of the AlGaN film is observed after the lift-off process, which is due to the relaxation of compressive stress in the film. Thus, separated AlGaN film is free from the disadvantages of using sapphire substrates such as the thermal mismatch, higher series resistance and poor heat dissipation so that it would enable high performance AlGaN-based ultraviolet (UV) light emitters.

This letter describes an extensive analysis of the time- and field-dependent trapping processes that occur in GaN-based gate injection transistors exposed to high drain voltage levels. Results indicate that-even if the devices do not suffer from current collapse-continuous exposure to high drain voltages can induce a remarkable increase in the on-resistance (Ron). The increase in Ron can be recovered by leaving the device in rest conditions: Temperature-dependent analysis indicates that the activation energy of the detrapping process is equal to 0.47 eV. By time-resolved electroluminescence characterization, we show that this effect is related to the capture of electrons in the gate-drain access region. Finally, we show that charge emission can be significantly accelerated through the injection of holes from the gate.

A normally-off AlGaN/GaN MOS heterojunction field-effect transistor (MOS-HFET) with a recessed gate structure formed by selective area regrowth is demonstrated. The fabricated MOS-HFET exhibits a threshold voltage of 1.7 V with an improved hysteresis of 0.5 V as compared with a device fabricated by a conventional dry etching process. An analysis of capacitance–voltage (C–V) characteristics reveals that the dry etching process increases interface state density and introduces an additional discrete trap. The use of the selective area regrowth technique effectively suppresses such degradation, avoiding the MOS interface from being exposed to dry etching. The results presented in this paper indicate that the selective area regrowth technique is promising for the fabrication of normally-off AlGaN/GaN MOS-HFETs.

Important clues for achieving well-behaved AlGaN/GaN metal-oxide-semiconductor (MOS) devices with Al-based gate dielectrics were systematically investigated on the basis of electrical and physical characterizations. We found that low-temperature deposition of alumina insulators on AlGaN surfaces is crucial to improve the interface quality, thermal stability, and variability of MOS devices by suppressing Ga diffusion into the gate oxides. Moreover, aluminum oxynitride grown in a reactive nitric atmosphere was proven to expand the optimal process window that would improve the interface quality and to enhance immunity against charge injection into the gate dielectrics. The results constitute common guidelines for achieving high-performance and reliable AlGaN/GaN MOS devices.

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.

Recent advances of GaN Gate Injection Transistors (GITs) and their applications to power switching systems are reviewed. The GITs are fabricated on cost effective Si substrates in a large diameter up tp 6inch, which exhibit normally-off operations by the use of p-AlGaN over an AlGaN/GaN hetero-junction. The GIT is free from the current collapse up to 600V enabling reliable operation of the power switching systems. The inverter for motor drive using GITs enables high efficiency of 99.3% at 1.5kW which is higher than that by conventional Si-IGBT (Insulated Gate Bipolar Transistor). The GITs are also applied various circuits in a power supply taking advantages of the high frequency operations. A GaN-based LLC resonant converter used in isolated DC-DC converter enables 1MHz operation with high efficiency of 96.4%, while conventional Si devices cannot be operated at such high frequencies. The above results demonstrate very promising potentials of high-voltage normally-off GaN GITs.