RF, Microwave, and Millimeter Wave Devices and Circuits and Their Applications

1. Introduction

The recent progress in the development of cost-effective, compact, and highly integrated high-frequency circuits in the RF, microwave, and millimeter-wave domains has significantly broadened the scope of these technologies across both traditional and emerging application areas. While RF and microwave systems were historically confined to military and aerospace use due to their complexity and high cost [1], advances in semiconductor fabrication technologies, such as CMOS and SiGe BiCMOS, have drastically reduced their footprint and cost, allowing their deployment in consumer-grade electronics, automotive systems, healthcare devices, and the burgeoning Internet of Things (IoT) ecosystem [2,3].

This Special Issue of Electronics, titled “RF, Microwave, and Millimeter Wave Devices and Circuits and Their Applications”, presents a comprehensive overview of recent innovations in the design of high-frequency systems and their multidisciplinary applications. The selected contributions span a wide array of domains—from power amplifiers, mixers, and phase shifters to advanced antenna arrays, metamaterial-inspired filters, etc.—demonstrating how the latest breakthroughs in circuit topology, device architecture, and packaging techniques are enabling high-performance systems at frequencies ranging from a few to tens of gigahertz.

The rapid expansion of wearable sensing and imaging systems in particular, empowered by conformal and miniaturized RF components, is opening up new avenues in personalized healthcare, biomedical diagnostics, and smart textiles [4,5]. Similarly, the increasing availability of the millimeter-wave spectrum (e.g., 28 GHz, 60 GHz) has led to explosive growth in high-data-rate communication systems for 5G and beyond, requiring innovations in front-end circuits, phased arrays, and beamforming architectures [6]. Other topics featured in this issue include wireless power transfer, non-contact sensors for industrial automation, energy-efficient radar systems, precision time-domain metrology, and reconfigurable systems using RF micro-electromechanical systems (MEMS) switches [7,8].

Furthermore, this Special Issue sheds light on the growing role of artificial intelligence and machine learning in the design and adaptation of RF/microwave circuits, such as for real-time link prediction in dynamic mmWave environments. With predictive modeling and symbolic regression, smart RF systems can now operate more efficiently and robustly in complex, multi-link settings. The fusion of circuit design, signal processing, and intelligent control represents a key trend toward self-optimizing and context-aware RF systems.

In summarizing these contributions, this issue not only highlights the breadth of current innovation but also points to critical emerging challenges. These include the need for better integration between analog and digital domains, improved thermal management in mmWave systems, robust packaging for harsh environments, etc. As we stand at the convergence of wireless connectivity, sensing, and computation, the opportunities for future research and impactful applications are vast.

We hope that this Special Issue will serve as both a valuable reference and a source of inspiration for engineers, researchers, and industry practitioners working at the frontier of RF, microwave, and millimeter-wave technologies.

2. Summary of the Contributions

We proudly present this compendium of 10 impactful papers that span advanced circuit design, component innovation, and applied systems in high-frequency electronics. The contributions span a wide range of applications, including (1) high-frequency CMOS and mixed-signal ICs, (2) filters, phase shifters, and signal conditioning, (3) antennas and propagation structures, (4) hybrid RF-AI systems, and (5) microwave imaging.

Related to the first topic, Yoo and Byeon present a 60 GHz CMOS down-conversion mixer realized in a 65 nm process. Their design integrates an LO buffer and transconductance stage with a transformer network that actively suppresses second harmonics. The resulting circuit achieves a conversion gain of ~6 dB, a noise figure around 6 dB, strong harmonic suppression (−26 dBc), and high linearity, all while consuming just 7 mW within a compact 0.51 mm² die. This positions it as a prime candidate for energy-efficient 5G receiver front-ends. In the second article, Kim and Byeon developed a high-performance 60 GHz CMOS power amplifier featuring both neutralization capacitors and compensation inductors. By mitigating the feedback and frequency-dependent gain roll-off inherent to mmWave PAs, they achieved over 13.4 dBm output power and more than 18 dB of gain, while preserving unconditional stability across the operating band. This advancement offers a robust mmWave transmitter solution suitable for compact wireless platforms. In the third contribution, Tripoli et al. designed a quad-core voltage-controlled oscillator using interstacked transformers to share bias and tank circuitry efficiently. It is an ideal candidate for integration in frequency synthesizers and local oscillators in mmWave communication systems.

In relation to the second topic, Ma et al. proposed a compact Low-Temperature Co-fired Ceramic (LTCC) band-pass filter. Despite its small size, it achieved greater than 44 dB stopband rejection and a smooth wideband response, which makes it well suited for RF front-end modules in IoT, handheld, and wearable devices. In the fifth contribution, Wang et al. explored ultra-wideband polyphase filter designs in CMOS, focusing on mitigating phase mismatches and insertion loss through adaptive biasing techniques. Their measured phase error remained below 0.3° and the amplitude mismatch was within 0.098 dB across the 2 to 8 GHz range, ensuring precise quadrature outputs vital for high-fidelity modulation in communications systems. In the sixth contribution, Jang and Park presented an L-C-L T-network phase shifter optimized for wideband performance in phased-array applications. The design demonstrates phase error under 1° over the 21.5 GHz to 40.0 GHz range, a flat amplitude response, and low insertion loss across its band, enabling precise beam steering with minimal signal degradation.

In relation to the third topic, Dzagbletey and Chung described a 28 GHz choke-ring antenna exhibiting a broad 60° half-power beamwidth and cross-polarization suppression below 28 dB. These properties make it highly suitable for mmWave applications involving precise dosimetry measurements and beamforming calibration in both research and deployment scenarios. In the eighth contribution, Zhao et al. delivered a detailed time-domain study of on-chip coplanar waveguide structures, exploring how sub-micron geometric variations affect amplitude and phase fidelity at mmWave frequencies. Their simulation-based work underscores the importance of fabrication precision for high-performance interconnects and chip-level signal integrity.

In relation to the fourth topic, Pendyala and Patil examined the prediction of mmWave channel quality across multiple simultaneous links using a hybrid of liquid time-constant networks, LSTMs, and symbolic regression. This combination significantly reduces prediction error from ~3.4 dB to ~0.25 dB while providing interpretability to the resulting models, thus offering adaptive insight for beam steering and network adaptation.

Lastly, in the tenth contribution, Heydari and Amineh introduced a groundbreaking near-field microwave imaging system that arranges transmit–receive antenna elements azimuthally around a target volume. This configuration eliminates the need for time-consuming mechanical scanning along the azimuthal direction while achieving a comparable spatial resolution. The authors experimentally demonstrate that by using a 360° antenna distribution, the acquisition time can be reduced by more than 7-fold without sacrificing image clarity, paving the way for real-time biomedical monitoring and rapid non-destructive testing.

3. Conclusions

This Special Issue showcases the progression of high-frequency electronics—from front-end CMOS ICs (mixers, PAs, VCOs) to advanced filtering networks, antennas, AI-enhanced communication systems, and microwave imaging. The research within contributes to advancing performance, miniaturization, and reconfigurability, presenting a clear trajectory toward 5G/6G communications, IoT sensing, biomedical wearables, and high-precision measurement systems.

We thank all the authors for their ingenuity, the reviewers for their rigorous evaluations, and the Electronics editorial team for their unwavering support. It is our hope that this issue will serve as a catalyst for future research and technological breakthroughs in the RF, microwave, and millimeter-wave community.