Ultra-high bandwidth, negligible latency and seamless communication are envisioned as milestones that will revolutionize the way by which societies create, distribute and consume information. The remarkable expansion of wireless data traffic has advocated the investigation of suitable regimes in the radio spectrum to satisfy users’ escalating requirements and allow the exploitation of massive capacity and massive connectivity. To this end, the Terahertz (THz) frequency band (0.1-10 THz) has received noticeable attention in the research community as an ideal choice for scenarios involving high-speed transmission. As such, in this work, we present an up-to-date review paper to analyze key concepts associated with the THz system architecture. THz generation methods are first addressed by highlighting the recent progress in the devices technology. Moreover, the recently proposed channel models available for propagation at THz band frequencies are introduced. A comprehensive comparison is then presented between the THz wireless communication and its other contenders. In addition, several applications of THz communication are discussed taking into account various scales. Further, we highlight the milestones achieved regarding THz standardization activities. Finally, a future outlook is provided by presenting and envisaging several potential use cases and attempts to guide the deployment of the THz frequency band.

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Terahertz Band: The Last Piece of RF Spectrum Puzzle for Communication Systems

Traditional terahertz (THz) equipment faces major obstacles in providing the system cost and compactness necessary for widespread deployment of THz applications. Because of this, the field of THz integrated circuit (THz IC) design in CMOS and SiGe HBT technologies has surged in the last decade. An interplay of advances in silicon process technology, design technique, and microelectronic packaging promises to narrow the gap between the requirements and the reality of system cost and performance of THz components. Furthermore, the scalability, reconfigurability, and signal processing features of silicon technology have initiated research in complex THz ICs that expand the functionality of THz systems; this has enabled new applications, methods, and algorithms. This paper reviews the progress in THz IC research and investigates several realizations of THz imaging and sensing applications with silicon-based components regarding their motivation, system performance, and challenges. THz computed tomography, broadband multicolor imaging, high-resolution FMCW radar imaging, subwavelength resolution near-field imaging, and compressed sensing are presented.

Terahertz Imaging and Sensing Applications With Silicon-Based Technologies

Highly scaled indium phosphide (InP) heterojunction bipolar transistor (HBT) technologies have been demonstrated with maximum frequencies of oscillation ( $f_{\max}$ ) of >1 THz and circuit operation has been extended into the lower end of the terahertz (THz) frequency band. InP HBTs offer high radio-frequency (RF) output power density, millivolt (mV) threshold uniformity, and high levels of integration. Integration with multilevel thin-film wiring permits the realization of compact and complex THz monolithic integrated circuits (TMICs). Circuit results reported from InP HBT technologies include: 200-mW power amplifiers at 210 GHz, 670-GHz amplifiers and fundamental oscillators, and fully integrated 600-GHz transmitter circuits. We review the state of the art in THz-capable InP HBT devices and integrated circuit (IC) technologies. Challenges in extending transistor bandwidth and in circuit design at THz frequencies will also be addressed.

InP HBT Technologies for THz Integrated Circuits

Terahertz radiation has significant potential in medical diagnosis and treatment because its frequency range corresponds to the characteristic energy of biomolecular motion. Advantageously, terahertz-specific low energy does not cause the ionization of biomolecules. In this paper, we review several state-of-the-art terahertz biomedical techniques and results and suggest potential techniques that may be applicable in real-world clinics in the near future. First, some techniques for enhancing the penetration depth into wet biological tissues are surveyed. Endoscopy and otoscopy methods for approaching internal organs are then discussed. The operation principles of sensors utilizing terahertz radiation are explained, and certain sensing examples related to blood disorders, diabetes, and breathing conditions are presented. The greatest potential of terahertz radiation in biomedical applications so far has been in cancer imaging, because terahertz radiation is ideal for measuring the superficial soft tissues in which most cancers occur. The examples presented herein include skin, oral, gastric, breast, and brain cancers. In search of a cancer-specific signal using terahertz radiation, methylated malignant DNA has been found to exhibit a characteristic resonance at approximately 1.65 THz. This resonance may help treat cancer through the demethylation of malignant DNA using high-power terahertz irradiation at this specific frequency, as well as serving as a potential cancer biomarker.

Potential clinical applications of terahertz radiation

A single-chip CMOS transceiver (TRX) capable of wireless data rates up to 80 Gb/s using part of frequencies (252–279 GHz) covered by IEEE Std 802.15.3d is presented. The TRX chip operates in either transmitter (TX) or receiver (RX) mode at frequencies comparable to ${f_{\mathrm {max}}}$ of the NMOSFET. The TX part adopts mixer-last architecture with four-way power combining using a ring circuit called a double-rat-race. The RX part adopts fundamental-mixer-first direct-conversion architecture. In the RX mode, the TX serves as an LO multiplier chain, which conventionally accounted for a significant part of the RX die area. The double-rat-race, having an improved design than the original one, integrates the TX and RX and also rejects unwanted harmonics generated by the frequency-doubler-based upconversion mixer. Low-loss, low-characteristic-impedance transmission lines are used extensively to combat losses. The TRX was fabricated using a 40-nm CMOS process. The saturated output power of the TX is −1.6 dBm at 265.68 GHz. The mean single-sideband noise figure (SSB NF) of the RX is 22.9 dB. The TX mode and the RX mode consume dc power of 890 and 897 mW, respectively. A wireless data rate of 80 Gb/s between a pair of TRX chips is demonstrated with 16QAM over a distance of 3 cm.

An 80-Gb/s 300-GHz-Band Single-Chip CMOS Transceiver

A 300 GHz integrated heterodyne receiver and transmitter for wideband communication and imaging applications have been developed in a 250 nm InP double-heterojunction bipolar transistor (DHBT) process. The receiver integrates a 300 GHz RF amplifier with a balun, a down-conversion mixer with an IF amplifier, and a local oscillator, all on a single chip. The transmitter is composed of the identical circuit blocks of RF amplifier and oscillator in addition to an up-conversion mixer. Compared to previous integrated receivers and transmitters reported at above 200 GHz, the proposed work includes the on-chip local oscillator and mixers operating at a fundamental mode. This simplifies the system architecture, thus not only reducing the chip area and DC consumption but also improving the RF performance such as high conversion gain, low spurious levels, and low noise figure. The receiver exhibits a peak conversion gain of 26 dB at 298 GHz, 3-dB bandwidth of 20 GHz, and noise figure of 12.0–16.3 dB at IF frequency from 1.1 to 7.7 GHz. The transmitter exhibits peak conversion gain of 25 dB, 3 dB bandwidth of 18 GHz, and output power of ${-}{\hbox{2.3}}$ dBm. The DC power consumption of the receiver and transmitter are 482 and 452 mW, respectively.

300 GHz Integrated Heterodyne Receiver and Transmitter With On-Chip Fundamental Local Oscillator and Mixers

Two fundamental-mode oscillators operating around 300 GHz, a fixed-frequency oscillator and a voltage-controlled oscillator (VCO), have been developed in this work based on a 250-nm InP heterojunction bipolar transistor (HBT) technology. Both oscillators adopted the common-base configuration for the cross-coupled oscillator core, providing higher oscillation frequency compared to the conventional common-emitter cross-coupled topology. The fabricated fixed-frequency oscillator and the VCO exhibited oscillation frequency of 305.8 GHz and 298.1-316.1 GHz (18-GHz tuning range) at dc power dissipation of 87.4 and 88.1 mW, respectively. The phase noise of the fixed-frequency oscillator was measured to be -116.5 dBc/Hz at 10 MHz offset. The peak output power of 5.3 dBm (3.8% dc-to-RF efficiency) and 4.7 dBm (3.2% dc-to-RF efficiency) were respectively achieved for the two oscillators, which are the highest reported power for a transistor-based single oscillator beyond 200 GHz.

300-GHz InP HBT Oscillators Based on Common-Base Cross-Coupled Topology

300-GHz direct and heterodyne imagers based on a 0.13-μm SiGe HBT technology were developed for active imaging applications in this work. The direct imager, which is based on the square-law principle, shows a maximum responsivity of 6121 V/W and a minimum noise equivalent power (NEP) of 21.2 pW/Hz1/2 at 315 GHz. The heterodyne imager, which consists of a mixer, a local oscillator, an IF amplifier, and an IF detector, exhibits a maximum responsivity of 322 kV/W and a minimum NEP of 3.9 pW/Hz1/2 at 300 GHz. Total dc power consumption of the direct imager is 0.6 mW, while the heterodyne imager consumes 21 mW. The chip areas of the direct and heterodyne imagers including the on-chip antenna are 460 × 410 and 610 × 610 μm2, respectively. To compare the performance of the two types of imagers for imaging applications, images from both imagers were acquired and compared with various output power levels of the signal source. It was demonstrated that the heterodyne imager shows much better image quality, especially when the signal source power is not sufficiently high.

300-GHz Direct and Heterodyne Active Imagers Based on 0.13-μm SiGe HBT Technology

This article presents two broadband frequency multiplier chains (FMCs) fabricated with a standard 130-nm SiGe BiCMOS process. In both solutions, a broadband push-push frequency doubler (x2) operating at 220–360 GHz was employed. In order to properly drive such a doubler, with sufficient input power and bandwidth, two different power amplifiers (PAs) have been adopted: the former is based on a cascode configuration and the latter is based on a stacked topology. In addition, a D-band frequency quadrupler (x4) has been integrated before the PAs and doubler, to complete the design of frequency eighth tupler (x8) chains. The first FMC with the cascode PA achieves peak output power of 2.3 dBm, with maximum conversion gain (GC) and bandwidth of 15 dB and 127 GHz, respectively. The second FMC with the stacked PA delivers a saturated output power of 2.5 dBm, with a maximum GC and a bandwidth of 12 dB and 72 GHz, respectively. The two FMCs consume maximum dc power of 0.537 and 0.542 W. The complete design procedure of the FMCs is presented in this article. To the best of our knowledge, the reported bandwidth is state-of-the-art and widest among SiGe based FMCs.

220–360-GHz Broadband Frequency Multiplier Chains (x8) in 130-nm BiCMOS Technology

This paper presents the development of an oscillator and a detector based on InP HBT technology operating near 300 GHz, and their application to terahertz reflection-mode imaging. The fabricated fundamental-mode common-base (CB) cross-coupled oscillator exhibits a peak output power of 5.3 dBm at 305.8 GHz, and the CB direct detector shows a responsivity higher than 40 kV/W and noise equivalent power lower than ${\text{35 pW/}}\sqrt {{\text{Hz}}} $ for a frequency range of 270–345 GHz. The imaging system employing the fabricated circuits as the signal source and detector was characterized for the resolution and dynamic range, and then, successfully applied for imaging various biological samples. The results show that low-power terahertz reflection-mode imaging based on solid-state electronic sources and detectors based on a commercial semiconductor technology is promising for various biomedical imaging applications.

Jongwon Yun

Papers

300 GHz Integrated Heterodyne Receiver and Transmitter With On-Chip Fundamental Local Oscillator and Mixers

300-GHz InP HBT Oscillators Based on Common-Base Cross-Coupled Topology

300-GHz Direct and Heterodyne Active Imagers Based on 0.13-μm SiGe HBT Technology

220–360-GHz Broadband Frequency Multiplier Chains (x8) in 130-nm BiCMOS Technology

Terahertz Reflection-Mode Biological Imaging Based on InP HBT Source and Detector