Publisher：IEEE Microwave and Wireless Technology Letters  

We have demonstrated high-efficiency rectification characteristics of a diode using a AlGaN/GaN high-electron mobility transistor (HEMT) for microwave wireless power transfer (WPT). For diode operation of the device, the gate electrode and one of the ohmic electrodes of a normally off AlGaN/GaN HEMT were short-circuited, which was called as a gated-anode diode (GAD). The fabricated GAD showed a high current density of 390 mA/mm with a low turn-on voltage of +0.6 V and a high breakdown voltage over 75 V. The GAD exhibited a 2.4-GHz RF-to-DC conversion efficiency of 96% at an input power of 23 dBm, where the inputs were tuned up to third-order harmonic frequencies using an automated tuner. This result represents state-of-the-art efficiency performance in rectifiers at the 2.4-GHz band.

DOI： 10.1109/LMWT.2024.3522057

Scopus

Publisher：Asia Pacific Microwave Conference Proceedings APMC  

This study presents a comparative evaluation of the high-frequency performance of single-gate (SG) and dual-gate (DG) HEMTs fabricated on GaN-on-GaN substrates. Comprehensive pulsed and continuous-wave (CW) S-parameter measurements reveal that the dual-gate architecture, designed for superior trap suppression, delivers significantly enhanced performance compared to conventional single-gate designs. Load-pull measurements further indicate that DG-HEMTs achieve higher power-added efficiency (PAE) at lower input power levels, whereas conventional SG-HEMTs require greater input power to reach peak PAE, which indicates a delay in saturation onset. Overall, these findings demonstrate the superior performance of the DG architecture and highlight its potential for high-PAE RF applications.

DOI： 10.1109/APMC65046.2025.11379693

Scopus

Publisher：Asia Pacific Microwave Conference Proceedings APMC  

Thin films of β-Ga<inf>2</inf>O<inf>3</inf>, one of the ultra-wide bandgap power semiconductors of β-Ga<inf>2</inf>O<inf>3</inf>, which are attracting attention as the next-generation devices, were epitaxially grown on β-Ga<inf>2</inf>O<inf>3</inf> substrates using the cost-effective mist chemical vapor deposition (CVD) method. A MESFET with a gate length of 0.45 μm and a gate width of 200 μm was fabricated on the substrate. To match the high impedance of the input and output, an amplifier circuit was constructed with a matching circuit connected directly to the high-frequency probe. A gain of 6.5 dB, a saturated output power of 12 dBm, and an added efficiency of 10% were obtained in the 2.4 GHz band. These results suggest that the mist CVD method can potentially be used for epitaxial growth in high-frequency devices at low cost.

DOI： 10.1109/APMC65046.2025.11378554

Scopus

Publisher：Journal of Computational Electronics  

AlGaN/GaN HEMT-based gated-anode diode (GAD) has been investigated with a physics-based TCAD simulation tool to understand its electrical transport characteristics. The simulation study predicted that the GAD exhibited low turn-on voltage (V<inf>on</inf> = + 0.77 V) over a conventional Schottky barrier diode (SBD). However, the GAD suffers from low breakdown voltage (V<inf>BD</inf>) because of strong electric field crowding at the gate edge. On the other hand, a δ-doped GaN cap (δ-DGC) layer has been able to spread out the electric field along the channel. With such modification in the epi-structure, a V<inf>BD</inf> of ~ 335 V could be achieved with the gated-anode-to-cathode distance (L<inf>gac</inf>) of 10 μm. TCAD-based RF simulation and small-signal S-parameter analysis were carried out to evaluate the expected RF performance of the GADs. From the transient response of the extracted small-signal equivalent circuit parameters, the cut-off frequency (f<inf>c</inf>) of the GADs with δ-DGC layer was 35.6 GHz at the exact turn-on condition (V<inf>on</inf>) of the device.

DOI： 10.1007/s10825-024-02226-w

Scopus

Publisher：IEEE Transactions on Electron Devices  

This article reports a comparative study on the effect of dopants in semi-insulating GaN substrates grown by hydride vapor phase epitaxy (HVPE) on the electrical characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs). GaN-HEMTs were fabricated on GaN substrates doped with iron (Fe), carbon (C), or manganese (Mn), and their electrical characteristics from 300 to 600 K were compared. Similar on-state characteristics were observed at room temperature, regardless of the substrate dopant. However, in the off-state breakdown characteristics, the devices on the C- and Mn-doped substrates showed slightly lower gate leakage currents than those on the Fe-doped substrate. Pulsed {I} -{V} characteristics showed that all devices had a small current collapse of approximately 10% or less, although the devices on the C- and Mn-doped substrates had slightly larger current collapse than that on the Fe-doped substrate. Furthermore, the temperature dependence of the off-state breakdown characteristics showed that the devices on the C- and Mn-doped substrates had a breakdown voltage ({V}-{text {BD}} ) that exceeded 100 V, even at 600 K, while that for the device on the Fe-doped substrate was less than 20 V. Arrhenius plots of leakage current suggested that for the devices on the Fe-doped substrate, current conduction through the substrate occurred above 350 K, which resulted in a low {V}-{text {BD}}. The devices on the C- and Mn-doped substrates had low leakage through the substrate due to the high resistivity of the substrate. These results indicate the feasibility of C- or Mn-doped GaN as HEMT substrates for use in high-voltage and high-temperature applications.

DOI： 10.1109/TED.2024.3375837

Scopus

Publisher：IEEE Transactions on Electron Devices  

AlGaN/GaN gated-anode diodes (GADs) have been proposed as microwave rectifying devices for microwave wireless power transmission (WPT) systems. However, achieving the desired turn-on voltage (V<inf>on</inf>) has posed a critical challenge due to the requirement for precise etching control. This article introduces a novel GAD structure aimed at enhancing productivity. In this new configuration, a GaN etch endpoint detection layer (GaN marker layer) is incorporated into the AlGaN layer, positioned approximately 4.5 nm away from the channel interface. During gate recess etching, the residual thickness of AlGaN is monitored using an optical interferometer. Etching is halted successfully upon detection of the GaN marker layer, signaled by an abrupt change in interference. Fabricated GADs consistently achieved the target V<inf>on</inf> of +0.20 V, exhibiting improved uniformity compared to prior GADs lacking a GaN marker layer. These devices typically demonstrated a maximum forward current of 0.69 A/mm and a reverse breakdown voltage exceeding 100 V. Additionally, a bridge-type rectifier circuit employing four GADs was simulated using a SPICE model constructed from measured pulse I-V characteristics and S-parameters. A conversion efficiency of 84% was predicted with an input power of 25.6 W (8 W/mm) at 5.8 GHz.

DOI： 10.1109/TED.2024.3381570

Open Access

Scopus

Publisher：IEEE Transactions on Electron Devices  

This article examines the impact of gate length (Lg) on the dc, pulsed I-V, and small-signal characteristics of dual-gate (DG) AlGaN/GaN high electron mobility transistors (HEMTs). T-shaped DG structures with gate lengths ranging from 0.1 to 0.3μ m were fabricated using i-line stepper lithography and a thermal reflow technique. The short-channel effect was successfully eliminated in the DG devices, while single-gate (SG) devices fabricated on the same wafer were significantly affected by this phenomenon. The dependence of current collapse on Lg in the DG devices showed a reduction in the collapse by a factor of 5 or more compared to the SG devices. Furthermore, a 120-nm DG-HEMT exhibited a 10-dB reduction in the isolation and an extrapolated maximum oscillation frequency of 239 GHz, whereas a 110-nm SG-HEMT exhibited that of 146 GHz.

DOI： 10.1109/TED.2024.3430889

Scopus

Publisher：Applied Physics Express  

Mist CVD was applied to grow the β-Ga<inf>2</inf>O<inf>3</inf> channel layer of a MESFET on a semi-insulating β-Ga<inf>2</inf>O<inf>3</inf> (010) substrate. The mobility and carrier concentration of the channel layer were 80 cm² V^-1 s^-1 and 6.2 × 10¹⁷cm⁻³, respectively. The device exhibited a pinch-off characteristic with a threshold gate voltage of −9 V, and the maximum drain current was 240 mA mm⁻¹. The maximum transconductance was 46 mS mm⁻¹ and the on-resistance was 30 Ω mm. This device performance suggests that mist CVD is a potential growth technology capable of providing low-cost devices in the future.

DOI： 10.35848/1882-0786/acefa5

Open Access

Scopus

Publisher：Physica Status Solidi A Applications and Materials Science  

AlGaN/GaN high-electron-mobility transistor (HEMT)-based gated-anode diodes (GADs) for a 5.8 GHz rectenna application are proposed. An anode of the GAD is formed by connecting a gate and a drain of a normally off GaN HEMT. Herein, a wide recessed HEMT structure reported in the previous article is modified to a buried-type recessed gate HEMT structure. The gate-to-cathode distance is optimized to maximize device performance. Typical direct current (DC) characteristics of the HEMTs are a threshold voltage (V<inf>th</inf>) of +0.4 V and a maximum drain current (I<inf>max</inf>) of 450 mA mm⁻¹. GADs using the HEMTs show the characteristics of maximum forward current (I<inf>f</inf>) of 600 mA mm⁻¹ and reverse breakdown voltage (BV<inf>r</inf>) of more than 100 V. The GAD using the recessed gate structure HEMT is shown to significantly improve I<inf>f</inf> and BV<inf>r</inf> simultaneously compared to the previous work using the wide recessed gate structure. The rectifying performance of a bridge-type rectifier simulated with SPICE model of the GAD shows radio frequency (RF)–DC conversion efficiency of 84% and RF input power of 13 W at 5.8 GHz using four GADs with each gate width (W<inf>g</inf>) of 400 μm.

DOI： 10.1002/pssa.202200837

Open Access

Scopus

Publisher：Physica Status Solidi A Applications and Materials Science  

The metal stop laser drilling of GaN-on-GaN devices is demonstrated using a UV sub-nanosecond laser as a light source. By monitoring the Bremsstrahlung emission at the drilling point, metal stops with a precision higher than 1 μm are realized for vias with a depth of 100 μm. From in situ laser-induced breakdown spectroscopy measurements, it is shown that endpoint detection is realized with high signal-to-noise ratio owing to the difference in the emission process between the strongly excited semiconductor and the metal. Herein, a through-substrate electrode with a resistance of less than 5 mΩ on GaN-on-GaN high-electron-mobility transistor (HEMT) wafers is demonstrated. The fabrication of through-substrate electrodes by this technique provides a simple process that does not require lithography or other complex processes. This process is expected to be useful in the fabrication of future GaN-on-GaN devices, including very thin GaN-on-GaN HEMTs.

DOI： 10.1002/pssa.202200739

Open Access

Scopus

Publisher：Electronics Letters  

This article reports radio frequency characteristics of 150-nm gate aluminum gallium nitride (AlGaN)/gallium nitride (GaN) high electron mobility transistors (HEMTs) fabricated using i-line stepper lithography and a thermal reflow technique. The authors have developed two different gate structures that were a field-plated gate using the lift-off process and a Y-shaped gate using the ion-milling process. Fabricated HEMTs using these different gate structures exhibited nearly equivalent DC characteristics. The field-plated gate device showed a unity current gain cutoff frequency (f<inf>T</inf>) of 35 GHz and a maximum oscillation frequency (f<inf>max</inf>) of 106 GHz, while the Y-shaped gate device showed f<inf>T</inf> of 36 GHz and f<inf>max</inf> of 115 GHz. The equivalent circuit analysis indicated a decrease in the gate-drain capacitance and the drain conductance is responsible for the improved f<inf>max</inf> in the Y-shaped gate device.

DOI： 10.1049/ell2.12798

Open Access

Scopus

Publisher：Cs Mantech 2023 2023 International Conference on Compound Semiconductor Manufacturing Technology  

We have developed a technique to slice GaN substrates with reduced kerf loss and processing time with laser. This technique can also be applied to slicing wafers after device fabrication, allowing a thin device layer to be sliced off from the wafer. This means that the electrical and thermal resistance of the devices can be reduced while the base GaN substrate remains reusable. We believe that the use of both wafer and device processing with laser slicing will greatly reduce the cost of GaN substrates in devices. With this technique, we believe that we can realize the practical application of devices using GaN substrates.

Scopus

Publisher：Scientific Reports  

The original version of this Article contained a repeated error where the symbol for micrometres “μm” was incorrectly given as millimetres “mm” in the Introduction, Experiments, Results and discussion, Figure 2 legend and the Conclusion. This error has been corrected throughout the text. The original Article has been corrected.

DOI： 10.1038/s41598-022-12628-0

Open Access

Scopus

Publisher：Scientific Reports  

As a newly developed technique to slice GaN substrates, which are currently very expensive, with less loss, we previously reported a laser slicing technique in this journal. In the previous report, from the perspective of GaN substrate processing, we could only show that the GaN substrate could be sliced by a laser and that the sliced GaN substrate could be reused. In this study, we newly investigated the applicability of this method as a device fabrication process. We demonstrated the thinning of GaN-on-GaN high-electron-mobility transistors (HEMTs) using a laser slicing technique. Even when the HEMTs were thinned by laser slicing to a thickness of 50 mm after completing the fabrication process, no significant fracture was observed in these devices, and no adverse effects of laser-induced damage were observed on electrical characteristics. This means that the laser slicing process can be applied even after device fabrication. It can also be used as a completely new semiconductor process for fabricating thin devices with thicknesses on the order of 10 mm, while significantly reducing the consumption of GaN substrates.

DOI： 10.1038/s41598-022-10610-4

Open Access

Scopus

Publisher：Applied Physics Express  

This paper investigated electroluminescence (EL) in AlGaN/GaN high electric mobility transistors fabricated on a free-standing GaN substrate (GaN-on-GaN) with ones on a SiC substrate (GaN-on-SiC) as a reference. When a drain voltage (V <inf>ds</inf>) of the GaN-on-GaN was increased, the EL peak was kept beside the gate, indicating that the highest electric field region stayed in the vicinity of the gate. On the other hand, EL of the GaN-on-SiC shifted from the gate to the drain electrode under an increased V <inf>ds</inf>. Our results indicate that the high-electric-field tolerance of GaN-on-GaN is higher than that of GaN-on-SiC, indicating that GaN-on-GaN is more suitable for high-voltage operation.

DOI： 10.35848/1882-0786/ac8782

Scopus

Publisher：IEEE Transactions on Electron Devices  

This article reports a systematic study on the effects of the epitaxial layer structure on the electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on freestanding GaN substrates. First, GaN-on-GaN HEMTs were fabricated comprising channel and buffer layers unintentionally doped with Fe atoms those were diffused from the substrate. Their electrical characteristics were compared with GaN-on-SiC HEMTs. The tradeoff relation between maximum drain current and the breakdown characteristics was improved in GaN-on-GaN devices than in GaN-on-SiC devices. A small current collapse was observed in GaN-on-GaN devices despite a high Fe density in the channel. This suggested an influence of Fe diffusion on the frequency dispersion to be comparatively limited. Second, GaN-on-GaN HEMTs were fabricated using a thin undoped channel layer grown on Fe-doped GaN substrate directly or through an Fe diffusion stopper layer. Without the stopper layer, the buffer leakage was significantly increased by reducing the channel thickness, and a 100-nm-thick channel device showed nonpinch-off characteristics. On the other hand, samples with the stopper layer exhibited no buffer leakage and a complete pinchoff up to 200 V. In addition, a sample with an intentional C doping to the buffer layer showed a relatively large current collapse, suggesting a major contribution of C-doping to the frequency dispersion of GaN-on-GaN HEMTs.

DOI： 10.1109/TED.2021.3126270

Scopus

Publisher：IEEE Journal of the Electron Devices Society  

This paper investigates bias-dependence of electroluminescence (EL) in an AlGaN/GaN HEMT fabricated on a SiC substrate. The HEMT exhibited a low-intensity reddish EL at the gate electrode at a drain voltage (Vds) of 30 V, which was confirmed with combination of a top-side view using a CMOS sensor camera and a back-side view using a silicon-intensified CCD. As Vds was increased to 48 V, color change from low-intensity red to high-intensity white was accompanied with shift of the location from the gate to the drain edge. The changes in the EL are attributed to a shift of the high electric field from the gate to the drain electrode and a concentration of electric field near the drain edge.

DOI： 10.1109/JEDS.2022.3163379

Open Access

Scopus

Publisher：Electronics Letters  

This article reports a high throughput 150-nm-gate AlGaN/GaN high electron mobility transistor (HEMT) process using i-line stepper lithography and a thermal reflow technique. Optimizing thermal reflow conditions, fabrication of a 150-nm gate structure was successfully realized with the initial resist opening of 0.7 μm. AlGaN/GaN field-plated HEMTs were fabricated on a semi-insulating SiC substrate by using this process. In spite of unoptimized structures, fabricated 150-nm gate devices exhibited the maximum drain current of 0.65 A/mm and the gate-drain breakdown voltage exceeding 200 V. Based on cold HEMT extraction measurements, the average gate length of 187 nm and the standard deviation of 30 nm were obtained on a quarter 4-in. wafer.

DOI： 10.1049/ell2.12303

Open Access

Web of Science

Scopus

Publisher：Electronics Letters  

Here, a gated-anode diode (GAD) is proposed where an anode electrode is formed by connecting a gate electrode and an ohmic electrode of a normally-off GaN HEMT for a 5.8 GHz rectenna. A wide recessed gate GaN GADs were prepared and the recess length dependence of their electrical characteristics was investigated. Typical DC characteristics of the HEMTs are a threshold voltage (V<inf>th</inf>) of +0.3 V and a maximum drain current (I<inf>max</inf>) of 300 mA/mm. The GADs showed the characteristics of maximum forward current (I<inf>f</inf>) of 350 mA/mm, reverse breakdown voltage (BV<inf>r</inf>) of 40 V, and off-state capacitance (C<inf>off</inf>) of 0.28 pF/mm by using optimized recess length. We constructed SPICE model of the GADs. The SPICE simulation predicted a rectifier efficiency of 81% and a DC output power of 10 W for bridge type 5.8 GHz rectifier using four GADs with each gate width of 0.8 mm.

DOI： 10.1049/ell2.12269

Open Access

Web of Science

Scopus

Publisher：Applied Physics Express  

This letter reports an impact of SiN film stress on electroluminescence (EL) of AlGaN/GaN HEMTs. The EL color of HEMTs turned from high-intensity white to low-intensity red, as the SiN film stress was increased from -24.2 (compressive) to +11.5 MPa (tensile). The weak reddish EL from the HEMT with a film stress of +11.5 MPa turned bright whitish when a drain-to-source voltage (V ds) was increased. A transient I d response after biasing stress revealed that severe current collapse emerged in the HEMT with a film stress of -24.2 MPa. In addition, the current collapse in HEMTs increased with the increased V ds.

DOI： 10.35848/1882-0786/ac1f40

Web of Science

Scopus

Publisher：Electronics Letters  

This paper investigates the electron trapping behaviour after bias stress in GaN high-electron-mobility transistors (HEMTs) fabricated on GaN substrate. On-resistance (R<inf>ON</inf>) and electron-trap-induced threshold voltage shift (ΔV<inf>th</inf>) of GaN HEMTs on GaN substrate are determined by gate quiescent bias (V<inf>gq</inf>) and independent of drain quiescent bias (V<inf>dq</inf>). This result indicates that the current collapse of GaN HEMTs is mainly attributed to the electron injection in barrier layer under gate region. Moreover, time constants of electron emission are dependent on the V<inf>gq</inf>. At least two time constants (τ<inf>1</inf> and τ<inf>2</inf>) are found to exist in the HEMTs after being switched from an off-state (V<inf>gq</inf> ≤ −30 V, V<inf>dq</inf>= 0 V) to an open channel condition. The τ<inf>1</inf> and τ<inf>2</inf> continue to increase with increasing |V<inf>gq</inf>|. It is speculated that the presence of multi-trap energy states in barrier layer results in the quiescent bias-dependence of time constants.

DOI： 10.1049/ell2.12201

Open Access

Web of Science

Scopus

Language：Japanese   Publisher：Applied Physics Letters  

The leakage current caused by the Si pileup at the regrowth interface of AlGaN/GaN high electron mobility transistors (HEMTs) is significantly suppressed by the semi-insulating Mg-doped GaN layer. Mg is unintentionally doped and can be originated from the graphite susceptor of metal organic vapor phase epitaxy. Before regrowth of the AlGaN/GaN heterostructure, the GaN template is treated with hydrochloric acid (HCl) and hydrogen peroxide/potassium hydroxide (H2O2/KOH) solution to promote the adsorption of Mg on the GaN surface. The Mg-doped GaN channel layer is highly resistive due to the passivation of hydrogen. The p-n junction formed by the weakly p-type Mg-doped GaN layer and the n-type Si pileup layer depletes the excess electrons at the regrowth interface. As a result, the off-state drain leakage current of the HEMT device can be decreased by two orders of magnitude at 40 V.

DOI： 10.1063/5.0034584

Open Access

Web of Science

Scopus

Publisher：IEEE Transactions on Electron Devices  

This article reports a systematic study focused on the mechanical stress effect of field-plate dielectric film on the electric characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs). AlGaN/GaN HEMTs were fabricated on SiC substrates, where the stress of a SiN field-plate dielectric film ranged from -252 (compressive) to +26.5 (tensile) MPa. Si-rich and compressive SiN films exhibited a significant increase in the isolation leakage. On the other hand, relatively N-rich and tensile SiN films showed a significant increase in the gate leakage current of HEMTs. In addition, pulsed ${I}$ - ${V}$ measurements showed the suppression in the current collapse by increasing the tensile stress. Consequently, small current collapse and small gate leakage current were obtained simultaneously with good isolation, as the film stress was optimized.

DOI： 10.1109/TED.2020.3029540

Web of Science

Scopus

Publisher：Japanese Journal of Applied Physics  

This paper investigates the current collapse and transient response of a drain current (I<inf>d</inf>) following the transition to an off-state in GaN high-electron-mobility transistors (HEMTs) on SiC substrates. Within a short time (∼10⁻³ s.) after an I<inf>d</inf> transition, significant current collapse and delayed recovery are observed in GaN HEMTs with a long gate-to-drain distance (L<inf>gd</inf>). Moreover, at least two time constants (τ<inf>1</inf> and τ<inf>2</inf>) are found to exist in HEMTs with an L<inf>gd</inf> of over 4.0 μm. As the L<inf>gd</inf> becomes longer, the activation energy required for electron emission is increased from 0.46 to 0.65 eV, which causes the time constant (τ<inf>2</inf>) to rise. Furthermore, the period of the off-state affects the time constants of GaN HEMTs without a field plate (FP). An extension of the off-state leads to an increase in the activation energy of the de-trapping process, and τ<inf>1</inf> and τ<inf>2</inf> are increased from 13 to 21 ms and from 150 to 350 ms, respectively.

DOI： 10.35848/1347-4065/abb7e2

Scopus

Event date： 2023.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Fukuoka   Country：Japan  

Event date： 2022.10

Language：English   Presentation type：Oral presentation (general)  

Venue：Berlin   Country：Germany