In the past year, fifth-generation (5G) wireless technology has seen dramatic growth, spurred on by the continuing demand for faster data communications with lower latency. At the same time, many researchers argue that 5G will be inadequate in a short time, given the explosive growth of machine connectivity, such as the Internet-of-Things (IoT). This has prompted many to question what comes after 5G. The obvious answer is sixth-generation (6G), however, the substance of 6G is still very much undefined, leaving much to the imagination in terms of real-world implementation. What is clear, however, is that the next generation will likely involve the use of terahertz frequency (0.1–10 THz) electromagnetic waves. Here, we review recent research in terahertz wireless communications and technology, focusing on three broad topic classes: the terahertz channel, terahertz devices, and space-based terahertz system considerations. In all of these, we describe the nature of the research, the specific challenges involved, and current research findings. We conclude by providing a brief perspective on the path forward.

CMOS IC technology has become an affordable means for implementing capable systems operating at 300 GHz and above. CMOS circuits have been used to generate a signal up to 1.3 THz to detect both amplitude and phase of signals up to 1.2 THz, and to detect a signal amplitude up to ~10 THz. Additionally, a transmitter and a receiver operating up to ~300 GHz for electronic smelling using rotational spectroscopy, a 30-Gb/s 300-GHz QPSK transmitter for data communication with -6-dBm output power, and an 820 GHz imaging array fabricated in CMOS have been reported. These results, along with the CMOS circuit performance in the literature, and link analyses suggest that the electronics necessary for everyday life applications in the terahertz band can be affordably realized.

A quadrature LC-VCO incorporating passive coupling and broadband harmonic combining for frequency multiplication by 4, and NMOS switched variable inductors is fabricated in 65 nm bulk CMOS to generate signals at 85 to 127 GHz. The passive quadrature coupling bypasses the need for a broadband on-chip bias-T, while reducing power consumption, phase noise, and the theoretical conversion loss for the 4th order harmonic generation by 3 dB over the linear superposition. The 39% frequency tuning range is at least 4x higher than the other CMOS implementations with center frequency over 90 GHz. At power consumption of 30-45 mW from a 1.5 V power supply, the measured output power varies from -15 to -23 dBm and phase noise at 10 MHz offset varies from -108 to -102 dBc/Hz over the output frequency range. These are sufficient for use in millimeter wave rotational spectroscopy.

This paper presents the design, fabrication, and measurement results for a novel non-contact-type radio-frequency (RF) microelectromechanical systems switch for 24-GHz radar applications. The proposed switches are free from unavoidable microwelding and stiction problems in other contact types, which in turn guarantee high reliability and long lifetime. The developed switch is a capacitive shunt type using a variation of the capacitance between the signal line and ground lines. The capacitance is precisely regulated by comb-drive actuators. This concept is simple, but the design requires a large and precise mechanical motion. In addition, for a low-loss switch structure, an air bridge with a large air gap is required. Therefore, the selective silicon-on-insulator process, based on the sacrificial bulk micromachining process, is designed for this switch fabrication. First, large insulating supports are fabricated 60 ¿m below the wafer surface, and then, the RF switch is fabricated on these insulating supports. The measured actuation voltage is 25 V, and the actuation stroke is 25 ¿m. The switching times are 8 ms in the off to the on state and 200 ¿s in the on to the off state. In the RF characteristic measurements, the insertion loss without the long coplanar waveguide line loss is 0.29 dB and the isolation is 30.1 dB at 24 GHz. The bandwidth is relatively narrow, and the isolation is 25 dB or better in the range of 23.5-29 GHz. The reliability test for the switch was performed 10⁹ times with 18-mW RF power. We observed no mechanical failure or RF performance degradation. A power handling capacity of 0.9 W with a hot-switch condition was achieved.

A 300-GHz 30-Gbps QPSK transmitter is demonstrated in 65-nm CMOS. The transmitter consists of an on-chip multi-mode modulator, an injection locked quadrature oscillator, a 40-GHz bandwidth power amplifier with constant gain and group delay, a 4X frequency multiplier chain to generate a 165-GHz LO signal for a double balanced up-conversion mixer that generates the output at 300 GHz. The transmitter without equalization consumes 180mW with an energy efficiency of 6 pJ/bit.

Electromagnetic waves in the millimeter- and sub-millimeter-wave frequency ranges are used in fast-scan rotational spectroscopy to detect gas molecules and measure their concentrations [1]. This technique can be used for indoor air quality monitoring, detection of toxic gas leaks, breath analyses for monitoring bodily conditions and many others. This paper reports a 210-to-305GHz receiver (RX) front-end for a rotational spectrometer that achieves SSB noise figure (NF) of 13.9 to 19dB by incorporating an on-chip antenna with a reflector formed using a phase-compensated artificial magnetic conductor (PC-AMC) instead a metal reflector to improve the bandwidth of the antenna, a single balanced subharmonic mixer using a pair of floating-body NMOS anti-parallel diode pairs (FB-APDPs), a hybrid-based broadband port-isolation structure, and a 20-GHz low-noise intermediate frequency (IF) amplifier (Fig. 25.2.1). The front-end is fabricated in a 65nm CMOS process that supports isolated p-wells and 10 copper layers with a ~3μm thick 10 ^th metal layer.