Current collapse phenomenon is a well known obstacle in the AlGaN/GaN HEMTs. In order to improve the surface stability of HEMTs, we have investigated the SiN passivation film deposited by T-CVD, and we found that it improves both gate leakage current and current collapse phenomenon [1]. Moreover, we compared the T-CVD and PE-CVD passivation films, on high electric field DC and RF characteristics. We found that T-CVD SiN passivation film improves BVds-off by 30% because of the reduction of gate leakage current. It also improved ηd in the output power characteristics by load-pull measurement, which indicates the decrease of the current collapse phenomenon. Also we fabricated a multi-fingered 50 W-class AlGaN/GaN HEMT with T-CVD SiN passivation film and achieved 61.2% of high drain efficiency at frequency of 2.14 GHz, which was 3.6 points higher than that with PE-CVD SiN passivation film.

We have fabricated AlGaN/GaN HEMTs with a thin InGaN cap layer to implement normally-off HEMTs with a small extrinsic source resistance. The key idea is to employ the polarization-induced field in the InGaN cap layer, by which the conduction band is raised leading to the normally-off operation. Fabricated HEMT with an In0.2Ga0.8N cap layer with a thickness of 5 nm showed normally-off operation with a threshold voltage of 0.4 V and a maximum transconductance of 85 mS/mm for the device with a 1.9-µm-long gate. By etching-off the In0.2Ga0.8N cap layer at the region except under the gate using gate and ohmic electrodes as etching masks, the sheet resistance has decreased from 2.7 to 0.75 kΩ/, and the maximum transconductance has increased from 85 to 130 mS/mm due to a reduction of the extrinsic source resistance. The transconductance was increased from 130 to 145 mS/mm by annealing the devices at 250 for 20 minutes in a N2 atmosphere.

In AlGaN/GaN high electron mobility transistors (HEMTs), drain current reduction by current collapse phenomenon is a big obstacle for a high efficient operation of power amplifier application. In this study, we investigated the effects of SiN passivation film quality on the electrical characteristics of AlGaN/GaN HEMTs. First, we conducted some experiments to investigate the relationship between electrical characteristics of AlGaN/GaN HEMTs and various conditions of SiN passivation film by plasma enhanced chemical vapor deposition (PE-CVD). We found that both gate current leakage and current collapse were improved simultaneously by SiN passivation film deposited by optimized condition of NH3 and SiH4 gas flow. It is found that the critical parameter in the optimization is a IN-H/ ISi-H ratio measured by Fourier transforms infrared spectroscopy (FT-IR) spectra. Next, a thermal CVD SiN was applied to the passivation film to be investigated from the same point of view, because a thermal CVD SiN is well known to have good quality with low hydrogen content and high IN-H/ISi-H ratio. We confirmed that the thermal CVD SiN passivation could improve much further both of the gate leakage current and the current collapse in AlGaN/GaN-HEMTs. Furthermore, we tried to apply the thermal CVD SiN to the gate insulator in MIS (Metal Insulator Semiconductor) structure of AlGaN/GaN HEMTs. The thermal CVD SiN passivation was more suitable for the gate insulator than PE-CVD SiN passivation in a view of reducing current collapse phenomena. It could be believed that the thermal CVD SiN film is superior to the PE-CVD SiN film to achieve good passivation and gate insulator film for AlGaN/GaN HEMTs due to the low hydrogen content and the high IN-H/ISi-H ratio.

The GaAs industry has been growing immensely during recent years. This is mainly driven by the tremendous growth of the wireless communication market, which is still continuously growing. Additionally, an emerging mmW market with applications in automotive, defense and optoelectronics is further driving the demand for GaAs components. The two largest European GaAs fabrication companies, UMS and Filtronic are very well positioned to address the complete frequency range from 1 GHz up to 100 GHz for commercial, high volume low cost markets, as well as individual niche applications. An overview of the companies' structures, their processes and design capabilities and also their new product developments will be presented in this paper.

To reduce the gate leakage current of AlGaN/GaN HEMTs, we selected ITO/Ni/Au for Schottky electrodes and Schottky characteristics were compared with those of Ni/Au electrodes. ITO/Ni/Au and Ni/Au electrodes were deposited by vacuum evaporation and annealed at 350 in nitrogen atmosphere. From the I-V evaluation results of ITO/Ni/Au electrodes, forward and reverse leakage currents were reduced. Schottky characteristics of ITO/Ni/Au electrodes were also improved compared to these of Ni/Au electrodes. In addition, substantial decrease of leakage currents was confirmed after the annealing of HEMTs with ITO/Ni/Au electrodes. This may be explained that ITO/AlGaN interface state became lower by the annealing. By the temperature dependence of I-V curves, clear dependence was confirmed for the gates with ITO/Ni/Au electrodes. On the other hand, small dependence was observed for those with Ni/Au electrodes. From these results, tunnel leakage currents were dominant for the gates with Ni/Au electrode. Thermal emission current was dominant for the gates with ITO/Ni/Au electrode. The larger temperature dependence was caused that ITO/AlGaN interface states were smaller than those for Ni/Au electrode. It was suggested that suppressed AlGaN Schottky barrier thinning was caused by the surface defect donors, then tunneling leakage currents were decreased. We evaluated HEMT characteristics with ITO/Ni/Au electrode and Ni/Au electrode. Id max and Gm max were similar characteristics, but Vth with ITO/Ni/Au electrode was shifted +0.4 V than that with Ni/Au electrode due to the higher Schottky barrier. It was confirmed to have a good pinch-off currents and low gate leakage currents by ITO/Ni/Au electrodes.

The mechanism of current collapse of AlGaN/GaN heterojunction field-effect transistors (HFETs) was investigated by gate bias stress with and without illumination. It is clarified that there are two positions where negative charges accumulate, at the gate edge and in the bulk epi-layer. In the gate-edge mode, the charge comes either through the passivation film or the AlGaN layer, depending on the resistance of the films. Reduction of leakage current in the passivation film will be important to suppress the surface-related collapse.

We propose a protocol for implementing secure circuit evaluation (SCE) based on the threshold homomorphic ElGamal encryption scheme and present the implementation results of the protocol. To the best of knowledge of the authors, the proposed protocol is more efficient in terms of computational complexity than previously reported protocols. We also introduce applications using SCE and estimate their practicality based on the implementation results.

In AlGaN/GaN high electron mobility transistors (HEMTs), Si3N4 passivation film brings effective improvements in the current collapse phenomenon, however, the suppression of this phenomenon in a high voltage operation can not be achieved in only the Si3N4 deposition process. In order to solve this problem, we have demonstrated an NH3-plasma surface pretreatment in the chamber of plasma enhanced chemical vapor deposition (PE-CVD) just before Si3N4 deposition process. We found that the optimized NH3-plasma pretreatment could improve the current collapse as compared with only the Si3N4 deposition and an excessive pretreatment made it worse adversely in AlGaN/GaN-HEMTs. It was confirmed by Auger electron spectroscopy (AES) analysis that the optimized NH3-plasma pretreatment decreased the carbon contamination such as hydrocarbon on the AlGaN surface and the excessive pretreatment degraded the stoicheiometric composition of AlGaN surface.

Phase pure cubic (c-) GaN/AlGaN heterostructures on 3C-SiC free standing (001) substrates have successfully been developed. Almost complete (100%) phase pure c-GaN films are achieved with 2-nm surface roughness on 3C-SiC substrate and stoichiometric growth conditions. The polarization effect in c-GaN/AlGaN has been evaluated, based on measuring the transition energy of GaN/AlGaN quantum wells (QWs). It is demonstrated that the polarization electric fields are negligible small in c-GaN/AlGaN/3C-SiC compared with those of hexagonal (h-)GaN/AlGaN, 710 kV/cm for Al content x of 0.15, and 1.4 MV/cm for x of 0.25. A sheet carrier concentration of c-GaN/AlGaN heterojunction interface is estimated to 1.61012 cm-2, one order of magnitude smaller than that of h-GaN/AlGaN. The band diagrams of c-GaN/AlGaN HEMTs have been simulated to demonstrate the normally-off mode operation. The blocking voltage capability of GaN films was demonstrated with C-V measurement of Schottky diode test vehicle, and extrapolated higher than 600 V in c-GaN films at a doping level below 51015 cm-3, to show the possibility for high power electronics applications.

This investigation of the temperature and illumination effects on the AlGaN/GaN HFET threshold voltage shows that it shifts about -1 V under incandescent lamp or blue LED illumination, while almost no shift takes place under red LED illumination. The temperature coefficient for the threshold voltage shift is +3.44 mV/deg under the illuminations and +0.28 mV/deg in darkness. The threshold voltage variation can be attributed to a virtual back-gate effect caused by light-generated buffer layer potential variations. The expressions for the potential variation are derived using Shockley-Read-Hall (SRH) statistics and the Maxwell-Boltzmann distribution for the carriers and deep traps in the buffer layer. The expressions indicate that large photoresponses will occur when the electron concentration in the buffer layer is extremely small, that is, highly resistive. In semi-insulating substrates, the substrate potential varies so as to keep the trap occupation function constant. The sign and the magnitude of the threshold voltage variation are explained by the shift of the pinning energy calculated from the Fermi-Dirac distribution function.

Accumulation of non-equilibrium longitudinal optical (LO) phonons (termed hot phonons) is considered as a possible cause for limitation of frequency of operation of GaN-based high-electron-mobility transistors (HEMTs). The experimental data on noise temperature of hot electrons at a microwave frequency as a function of supplied electric power is used to extract information on hot phonons: the hot-phonon lifetime, the equivalent hot-phonon temperature, the effective occupancy of hot-phonon states involved into electron-LO-phonon interaction. The possible ways for controlling the hot-phonon effect on electron drift velocity through variation of electron density, channel composition, and hot-phonon lifetime are discussed. The expected dependence of hot-electron drift velocity on hot-phonon lifetime is confirmed experimentally. A self-consistent explanation of different frequency behaviour of InP-based and GaN-based HEMTs is obtained from a comparative study of hot-phonon effects.

Based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing (RTA), enhancement mode (E-mode) AlGaN/GaN HEMTs with low on-resistance and low knee-voltage were fabricated. The fabricated E-mode AlGaN/GaN HEMT with 1 µm-long gate exhibits a threshold voltage of 0.9 V, a knee-voltage of 2.2 V, a maximum drain current density of 310 mA/mm, a peak gm of 148 mS/mm, a current gain cutoff frequency fT of 10.1 GHz and a maximum oscillation frequency fmax of 34.3 GHz. In addition, the fluoride-based plasma treatment was also found to be effective in lowering the gate leakage current, in both forward and reverse bias. Two orders of magnitude reducation in gate leakage current was observed in the fabricated E-mode HEMTs compared to the conventional D-mode HEMTs without fluoride-based plasma treatment.

The correlation between ohmic contact resistivity (ρc) and transconductance (gm) in AlGaN/GaN high-electron-mobility transistors (HEMTs) was investigated. To characterize ρc precisely, we fabricated a circular transmission line model (c-TLM) pattern adjoined to a field-effect transistor (FET) pattern on an HEMT. By measuring ohmic contact resistance and sheet resistance using the adjoined c-TLM, intrinsic transconductance (gm0), which is not influenced by the source resistance, can be estimated. The gm0 thus obtained is between 179 and 206 mS/mm. Then, it became possible to calculate the correlation between gm and (ρc. We found that ρc should be below 10-5 Ωcm2 for the improvement of gm in AlGaN/GaN HEMT when Rsh 400 Ω/.

A model for the enhancement-mode operation of an AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistor (MIS-HFET) under DC and AC conditions is proposed. In DC operation at positive gate voltages, the MIS-HFET can be divided into a transistor area and a resistor area due to the diode nature of the insulator/AlGaN interface. The transistor area shrinks with the increases in gate voltage. The intrinsic-transistor gate-length reduction causes a drain current increase. The I-V characteristics based on the gradual channel approximation are derived. The ID hysteresis of the MIS-HFET is investigated by a circuit simulation using SPICE. We have confirmed that the hysteresis was caused by the phase difference between the potential variation of the gate insulator/AlGaN interface and that of the gate electrode due to CR components in the gate structure.

We analyzed passivation film and the AlGaN surface states using open-gated structures of AlGaN/GaN HFETs by numerical simulation and experiments. From the analyses, we confirmed that insulating film conductivity plays the prominent roles in device performances of the wide bandgap semiconductor device. Device simulation confirmed that the difference in ID-VG characteristics is due to the trapping type of the surface states; electron-trap type or hole-trap type. For electron-trap type surface states, the surface potential pinned at electron quasi-Fermi level, which is the same as the channel potential in the open-gated FETs. As a result, surface potential of ungated region is equal to the channel electric potential resulting in the uncontrollability of the channel current by the edge placed gate electrode. For hole-trap type surface states, the surface potential is pinned at hole quasi-Fermi level, which must be the same as the edge placed gate electrode potential. Then, the AlGaN surface potential varies with the electrode potential variation allowing the control of channel current as if the whole channel is covered with a metal electrode. Experiments for open-gated FET with unpassivated surface show no current variation. This corresponds to electron-trap type surface states from the simulation. On the other hand, SiOX evaporated open-gated FET show current control by the gate electrode. The ID-VG characteristics resembles in simulated ID-VG characteristics with hole-trap surface states. However, the estimated time constants for the trap reactions are incredibly long due to the deep energy level for the surface states in wide bandgap semiconductors. In addition, the open-gated FET showed reverse threshold shift to the value expected from the hole-trap pinning levels. So, we concluded that the no current variation for the unpassivated open-gated FET can be attributed to electron traps in the surface states, but the control of the drain current for SiOX deposited open-gated FET is not by surface hole-traps, but by slightly conductive passivation film of SiOX.

We propose a novel Schottky barrier diode with a dual Schottky structure combined with an AlGaN/GaN heterostructure. The purpose of this diode was to lower the on-state voltage and to maintain the high reverse breakdown voltage. An AlGaN/GaN heterostructure was grown using a metalorganic chemical vapor deposition (MOCVD). The Schottky barrier diode with a dual Schottky structure was fabricated on the AlGaN/GaN heterostructure. As a result, the on-voltage of the diode was below 0.1 V and the reverse breakdown voltage was over 350 V.

Normally, flow field is described with governing equations, such as the Navier-Stokes equations. However, for complex flow including multiphase and reactive flow such as combustion, this approach may not be suitable. As an alternative approach, Lattice Gas Automata (LGA) has been used to simulate fluid with mesoscopic particles by assuming that space and time are discrete, and the physical quantities take only a finite set of values. In this study, the model for combustion simulation is proposed, with the reaction probability depending on the local temperature to simplify the chemical reaction. Here, counter-flow twin flames are simulated. In order to validate this approach, some results of non-reactive flow are presented, compared with those by solving Navier-Stokes equations.

High-frequency characterization and delay-time analysis have been performed for a short channel AlGaN/GaN heterojunction FET. The fabricated device with a short gate length (Lg) of 0.07 µm exhibited an extrinsic current gain cutoff frequency of 81 GHz and a maximum frequency of oscillation of 190 GHz with a maximum stable gain (MSG) of 8.2 dB at 60 GHz. A new scheme for the delay-time analysis was proposed, in which the effects of rather large series resistance RS + RD are properly taken into account. By applying the new scheme to a device with Lg=0.25 µm, we obtained an effective high-field electron velocity of 1.75107 cm/s, which is consistent with our previous results calculated using Monte Carlo simulation.

In order to clarify the mechanism of gate leakage in AlGaN/GaN heterostructure field effect transistors (HFETs), temperature (T)-dependent current-voltage (I-V) characteristics of Ni/n-AlGaN Schottky contact were measured in detail. Large deviations from the thermionic emission transport were observed in I-V-T behavior with anomalously large reverse leakage currents. An analysis based on the thin surface barrier (TSB) model showed that the nitrogen-vacancy-related near-surface donors play a dominant role in the leakage through the AlGaN Schottky interface. As a practical scheme for suppressing the leakage currents, use of an insulated gate (IG) structure was investigated. As the insulator, Al2O3 was selected, and an Al2O3 IG structure was formed on the AlGaN/GaN heterostructure surface after an ECR-N2 plasma treatment. An in-situ XPS analysis exhibited successful formation of an ultrathin stoichiometric Al2O3 layer which has a large conduction band offset of 2.1 eV at the Al2O3/Al0.3Ga0.7N interface. The fabricated Al2O3 IG HFET achieved pronounced reduction of gate leakage, resulting in the good gate control of drain currents up to VGS = +3 V. The maximum drain saturation current and transconductance were 0.8 A/mm and 120 mS/mm, respectively. No current collapse was observed in the Al2O3 IG-HFETs, indicating a remarkable advantage of the present Al2O3-based insulated gate and passivation structure.

We present ultra low noise- and wide dynamic range performances of an AlGaN/GaN heterostructure FET (HFET). An HFET fabricated on a high quality epitaxial layers grown on a semi-insulating SiC substrate exhibited impressively low minimum noise figure (NF min ) of 0.4 dB with 16 dB associated gain at 2 GHz. The low NF (near NF min ) operation was possible in a wide drain bias voltage range, i.e. from 3 V to 15 V. At the same time, the device showed low distortion character as indicated by the high third order input intercept point (IIP3), +13 dBm. The excellent characteristics are attributed to three major factors: (1) high quality epitaxial layers that realized a high transconductance and very low buffer leakage current; (2) excellent device isolation made by selective thermal oxidation; (3) ultra low gate leakage current realized by Pd based gate. The results demonstrate that the AlGaN/GaN HFET is a strong candidate for front-end LNAs in various mobile communication systems where both the low noise and the wide dynamic range are required.