This paper presents recent advancements in device technology for sub-THz power amplifiers aimed at high-speed, high-capacity communication. To extend the communication range and reduce power consumption, we aimed to achieve high output power and high efficiency in transistors using compound semiconductors comprising GaN and InP materials. We demonstrated that techniques such as InAlGaN barrier layers and InGaN back barriers are effective in improving the characteristics of semi-mature 100-GHz band GaN-based HEMTs. Furthermore, by implementing a MOS structure in InP-based HEMTs, we achieved world-leading output power and efficiency at 300-GHz band.

This letter presents a four-way W-band power amplifier (PA) module that packages GaN-based high electron mobility transistor (HEMT) millimeter-wave monolithic integrated circuits (MMICs) designed with coplanar waveguides (CPWs). A resistive-back-metal (RBM) layer was formed on the back side of the MMICs to keep stable in the module even without using substrate vias. The verification of the effect of the RBM layer was performed using CPW-based MMIC test chips. The assembled PA module, where millimeter-wave components, such as a waveguide (WG) combiner/divider and a WG-to-microstrip line transition, were optimized to reduce losses and flatten frequency responses, demonstrated stable and broadband characteristics in a frequency range of 88-100 GHz. A peak output power (P-OUT) of 37.1 dBm with a power-added efficiency (PAE) of 9.9% was achieved at a frequency of 92 GHz and a bias voltage (V-DS) of 15 V. V-DS was set to 20 V to enhance the power performance of the PA module, achieving a peak P-OUT of 38.8 dBm with a PAE of 8.1%. To the best of our knowledge, the PA module exhibited the highest peak P-OUT value among W-band single PA modules with a bandwidth (BW) of more than 10 GHz.

An AlGaN/GaN high-electron-mobility transistor (HEMT) on a free-standing GaN substrate achieved impressive power-added and drain efficiencies of 85.2% and 89.0%, respectively, at 2.45 GHz. We improved the GaN channel quality by reducing the C concentration and eliminated the buffer leakage path by removing the residual Si at the substrate-epitaxial layer interface. These improvements, combined with the reduction in dislocation density and the elimination of the nucleation layer by using a free-standing GaN substrate, contributed to the enhanced efficiency. To the best of our knowledge, the achieved efficiency represents the highest reported for GaN-based discrete HEMTs in this frequency band.

This study describes high-power and high-efficiency W-band InAlGaN/AlN/GaN high-electron-mobility transistors (HEMTs) for future sub-terahertz wireless communications. A low-thermal-budget selective-area growth (SAG) process was developed to obtain low contact resistance with low trap states. Transmission lines and substrate structures were optimized to obtain high-thermal conductivity and low substrate resonance. Consequently, a high output power of 28.7 dBm (742 mW), output power density of 4.6 W mm-1, and power-added efficiency (PAE) of 28.0% were achieved with pre-matched InAlGaN/AlN/GaN HEMTs at 90 GHz, which were superior combination of output power and PAE compared to the conventional high-temperature SAG process.

This paper demonstrated high-output-power and high-efficiency power amplifier (PA) monolithic microwave-integrated circuit (MMIC) at 300-GHz band (252-296 GHz) with the use of InP-based metal-oxide-semiconductor high-electron-mobility transistors (HEMTs) with composite-channel (CC) and double-side-doping (DD) techniques. The CC-DD structure obtained high output current and low channel resistance due to the improved carrier density and mobility. W-band load-pull measurement revealed the drastically improved output power density of CC-DD structure compared with that of single-channel DD structure. The 2-stage cascaded, 4-way, and 16-way PA-MMICs were designed based on stacked common-gate transistors with current reuse topology. The cascaded PA-MMIC exhibited a power-added efficiency (PAE) of 7.8%, and the 16-way PA-MMIC exhibited an output power of 16.9 dBm. These values are the highest among all the values reported for the 300-GHz band PA-MMICs. The 4-way PA-MMIC achieved a high output power of 13.6-14.6 dBm and high PAE of 4.8%-6.3% simultaneously at the entire 300-GHz band.

Sub-THz has received considerable attention for 6G wireless communications due to its large available bandwidth. Sub-THz multiple-input multiple-output (MIMO) system is expected to implement 6G data rates with more than 100 Gbps. Nonlinear distortion has always been a challenge for wireless communications due to the nonlinear characteristics of the RF chain especially for power amplifiers (PAs). The highest data rates cannot be obtained without effective compensation of nonlinear characteristics of the whole transmission channel. In this paper, we demonstrate and experimentally validate a deep neural network (DNN) nonlinear equalizer (NLE) to effectively compensate for nonlinearities of PAs on the SC-FDE MIMO system. Using the proposed DNN-NLE, we successfully transmit a 256QAM-3/4 signal with 107.52 Gbps for 2×2 MIMO in a chip rate of 10.24 GHz at a carrier frequency of 96 GHz over 36 m.

A high-efficiency uniform/selective heating microwave oven was developed. Because the power amplifier requires high-efficiency characteristics to function as a microwave source, a free-standing Gallium Nitride (GaN) substrate was applied in this study. By applying a harmonic tuning circuit, an output power of 71 W and PAE of 73% were achieved in pulsed operation, and an output power of 63 W and PAE of 69% were achieved in CW operation. Moreover, we fabricated a prototype PA module that consists of an oscillator, a driver amplifier, PA, and other RF circuits. The output power was controlled by pulse width modulation to maintain high efficiency regardless of output power. We evaluated the arrangement of antenna polarizations to isolate each antenna. By suppressing the interference of output from adjacent antennas, it is possible to irradiate the object on the top surface of the antenna, thereby demonstrating heating characteristics with small temperature unevenness. The prototype microwave oven successfully demonstrated uniform/selective heating.

This work shows a broadband, high-efficiency power amplifier (PA) monolithic microwave integrated circuit (MMIC) that uses InP-based metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) with an extended drain-side access region and broadband conjugate matching topology. Advanced device technologies, namely, double-side-doped structures, MOS gate structures, and asymmetric gate recess, were adopted, and the length of the drain-side access region was optimized to simultaneously obtain high power and efficiency. A common-source PA MMIC based on InP-based MOS-HEMTs was fabricated, and an interstage circuit was designed to maximize the S21 per unit stage in the broadband, resulting in a record-high power-added efficiency and wide bandwidth.

For sub-THz communication applications, we developed an antenna array module with element pitch less than 1 wavelength (1λ) at 300 GHz. The 1 × 4 waveguide-based antenna array module showed an antenna gain of 10 dBi and a beam steering angle of ±10° with reduced grating lobes. In addition, we developed a hybrid-interposer structure (lower side: heat dissipation/upper side: transmission layers) for the heterogeneous integration of an antenna array with power amplifiers. As a result, low-loss sub-THz transmission and high-efficient thermal dissipation were achieved simultaneously.

In this study, we investigated the effect of oxidant sources on carbon-related impurities in atomic layer deposited (ALD)-Al 2 O 3 by focusing on the plasma-induced decomposition of –CH 3 groups which is attributed to ALD precursor. We found that C–O bonds were detected in ALD-Al 2 O 3 using O 2 plasma instead of the C–H bonds which is attributed to the –CH 3 groups of trimethylaluminum. It is considered that the decomposition of –CH 3 groups was enhanced by O 2 plasma, and C–O residue was generated. We concluded that the decomposition of –CH 3 groups by oxidant sources must be suppressed to reduce the carbon-related impurities in ALD-Al 2 O 3 .

Herein, we successfully improved the maximum oscillation frequency and maximum stable gain (MSG) across a wide bias range of surface-oxide-controlled (SOC) InGaAs/InAlAs inverted-type metal-oxide-semiconductor high-electron-mobility transistors (inverted MOS-HEMTs) by reducing the gate leakage current and drain conductance ( g d ). H 2 O vapor treatment selectively decreased the narrow band gap indium oxide at the surface of the In-based epitaxial layer via the SOC process before the gate oxide deposition. Furthermore, the calculation of band profiles indicated that the g d reduction was possibly attributed to the suppression of impact ionization in the InGaAs channel by inverted MOS-HEMTs. Consequently, SOC-inverted MOS-HEMTs demonstrated a high MSG of >12 dB at 100 GHz across a wide bias range.

In this study, we proposed a low-resistance and low-thermal-budget ohmic contact by introducing periodic microstructures for AlGaN/AlN/GaN high-electron-mobility transistors (HEMTs). Insertion of AlN spacer layers is effective in improving electron mobility but can degrade contact resistance, in general. An ohmic contact with periodic microstructures was fabricated through low-damage patterned recess etching and metallization via low-temperature annealing (600 °C). The optimization of structural parameters of periodic microstructures allowed a low contact resistance of 0.29 $\Omega \cdot $ mm, which is less than half of that of the conventional recessed ohmic contact prepared in this study. A simplified equivalent circuit model reproduced the experimental results and predicted that enhanced metal/channel direct contact areas contributed to the reduction of contact resistance. Pulsed current–voltage measurements revealed that an AlGaN/AlN/GaN HEMT with periodic microstructures showed considerably suppressed current collapse compared with that of a selective-area growth (SAG) ohmic contact, indicating that the introduction of trap states was suppressed in periodic-structured devices owing to low-thermal-budget processes. Under high off-stress conditions, periodic-structured devices showed the highest drain current at knee voltage compared to the devices with a conventional planar-recessed ohmic contact and an SAG ohmic contact. Therefore, ohmic contact with periodic microstructures is one of the best techniques to achieve low contact resistance with low-thermal-budget processes.

This study presents a broadband, high-efficiency power amplifier (PA) monolithic microwave integrated circuit (MMIC) that uses InP-based metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) with an extended drain-side access region and a broadband conjugate matching topology. Advanced device technologies such as double-side-doped structures, MOS gate structures, and asymmetric gate recess were adopted, and the length of the drain side access region was optimized to simultaneously obtain high power and efficiency. A common-source PA MMIC was fabricated, and an interstage circuit was designed to maximize the S21 per unit stage in the broadband, resulting in a record-high PAE and wide bandwidth.

In this study, we successfully achieved flat cutoff frequency (f (T)) and maximum oscillation frequency (f (max)) across a wide bias range on InP-based high-electron-mobility transistors (HEMTs) using a steam-annealed ultrathin-Al2O3 gate dielectric due to the reduction in gate leakage current and increase in forward breakdown voltage. Fourier transform infrared spectroscopy analysis demonstrated that steam annealing reduced defects in atomic layer deposited-Al2O3 because of the hydrolysis of carbon impurities and that reducing electron traps suppressed the increase in sheet resistance due to dielectric passivation. Consequently, the insulated-gate HEMTs developed show a high f (max) of > 700 GHz across a wide bias range.

In this study, we investigated the heat dissipation characteristics of gallium nitride (GaN) high-electron mobility transistors (HEMTs) fabricated on a freestanding GaN substrate (GaN-on-GaN HEMTs) by simulation. Although the GaN substrate has a lower thermal conductivity than the conventional silicon carbide (SiC) substrate, it was demonstrated that the GaN-on-GaN HEMT has a thermal resistance comparable to the conventional GaN-on-SiC HEMT in a simulated structure composed of a 100- $\mu \text{m}$ -thick substrate and a 2- $\mu \text{m}$ -thick GaN epitaxial layer. This is because GaN-on-GaN has a higher thermal conductivity of the GaN epitaxial layer and a lower thermal boundary resistance than GaN-on-SiC. Next, the thermal design of a 400-W/eight-channel selective heating microwave oven, which consists of eight pairs of patch antennas and power amplifier (PA) modules, involving a 50-W-class GaN-on-GaN HEMT PA, a driving amplifier, an oscillator, and several circuits, was carried out. The fan and fin specifications and enclosure configuration have been optimized to keep the temperature of the metal base on which the PA modules are mounted below 40 °C during full operation. Excellent agreement between the oven thermal simulation and the prototype thermocouple measurements was confirmed for the temperature difference between the metal base and the ambient. Finally, device dimensions for reliable operation were determined by simulating the channel temperature of the GaN-on-GaN HEMT PA during microwave heating. To keep the maximum channel temperature below 200 °C, the simulated device layout with a unit gate width ( $W_{\mathrm {gu}}$ ) of 100– $300~\mu \text{m}$ and a gate-to-gate spacing ( $L_{\mathrm {gg}}$ ) of 30– $70~\mu \text{m}$ was acceptable at a drain efficiency ( $\eta _{\mathrm {D}}$ ) of 75%. However, appropriate $W_{\mathrm {gu}}$ and $L_{\mathrm {gg}}$ were found to be required with a $\eta _{\mathrm {D}}$ of 65%.

†富士通株式会社,厚木市 Fujitsu Limited, 10–1 Morinosato-Wakamiya, Atsugi-shi, 243–0197 Japan ††株式会社富士通研究所,厚木市 Fujitsu Laboratories Ltd., 10–1 Morinosato-Wakamiya, Atsugi-shi, 243–0197 Japan a) E-mail: kumazaki.yusuke@fujitsu.com DOI:10.14923/transelej.2021JCI0004 ここで,φはドレイン電流の流通角,Vds はドレイン 電圧,Vmin は RF 動作時の最小ドレイン電圧であり, おおよそ knee 電圧 Vk に相当する.図 1 は φ = π/2 (B 級動作)として算出した ηd と Vds の関係であり, Vmin , 0 Vである限りは Vdsが大きくなるほど ηdは高 くなる.言い換えると,Vmin が変わらない限りは動作 電圧が高くなるほど高い効率を実現できる.ワイドバ