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In this article, a set of oscillators and a heterodyne image receiver operating near 600 GHz have been developed, each based on a 250-nm InP heterojunction bipolar transistor (HBT) technology and a 130-nm SiGe HBT technology, respectively. The oscillators are based on the common-base cross-coupled push–push topology, which employed coupled-line loads for improved output power and efficiency with a small area. The measured oscillation frequency ranges of the three oscillators with different coupled-line lengths were 628–682, 556–610, and 509–548 GHz, respectively, with a tuning capability achieved with bias variation. The maximum output power and the dc-to-RF efficiency achieved with the oscillators were −10 dBm and 0.19%, respectively. The heterodyne receiver, which consists of a mixer, an IF amplifier, and an IF detector, exhibited a maximum responsivity of 469 kV/W and a minimum noise equivalent power (NEP) of 0.64 pW/Hz1/2. A transmission-mode 3-D THz tomography imaging setup was established with one of the fabricated oscillators and the heterodyne receiver employed as the signal source and the image detector, respectively. With the imaging setup, a successful reconstruction of 3-D images of a target object was demonstrated based on the filtered back-projection algorithm.

A 300-GHz quadrature voltage-controlled oscillator (QVCO) integrated with a pair of mixers based on an InP heterojunction bipolar transistor (HBT) technology has been developed. A super-harmonic coupling technique was applied to generate the quadrature signal in the QVCO. The oscillation frequency tuning range is 24.8 GHz (from 286.1 to 310.9 GHz), and the frequency range maintaining the quadrature operation (within ±10°) is 4.8 GHz (297.2–302.0 GHz). The maximum output power of the QVCO was measured as 0.46 dBm at 286.3 GHz. The measured phase noise is −91.4 dBc/Hz with 10 MHz offset at 294.2 GHz. The integrated QVCO-mixer consumes a dc power of 167.5 mW.

This paper presents a wide tuning range push-push voltage-controlled oscillator (VCO) at the sub-THz frequency bands. With the introduction of a novel capacitive splitting feedback (CSF) technique and tunable source degeneration (TSD) network, the proposed VCO can simultaneously archive a wide tuning range while maintaining a high output power. The CSF technique improves the oscillation frequency by reducing the effective transistor capacitances, and a wide tuning range is achieved through the TSD network without employing varactors. The proposed VCO is fabricated in a 40-nm digital CMOS process. It shows a -3.5 dBm peak output power with 1.35% DC-to-RF efficiency and a wide tuning range of 10.16% at a center frequency of 267.2 GHz.

Spin Hall nano-oscillators (SHNOs) exploiting current-driven magnetization auto-oscillation have recently received much attention because of their potential for oscillator-based neuromorphic computing. Widespread neuromorphic application with SHNOs requires an energy-efficient way to tune oscillation frequency in broad ranges and store trained frequencies in SHNOs without the need for additional memory circuitry. Voltage control of oscillation frequency of SHNOs was experimentally demonstrated, but the voltage-driven frequency tuning was volatile and limited to megahertz ranges. Here, we show that the frequency of SHNO is controlled up to 2.1 GHz by a moderate electric field of 1.25 MV/cm. The large frequency tuning is attributed to the voltage-controlled magnetic anisotropy (VCMA) in a perpendicularly magnetized Ta/Pt/[Co/Ni]n/Co/AlOx structure. Moreover, non-volatile VCMA effect enables control of the cumulative frequency using repetitive voltage pulses, which mimic the potentiation and depression functions of biological synapses. Our results suggest that the voltage-driven frequency tuning of SHNOs facilitates the development of energy-efficient spin-based neuromorphic devices.

In this work, a 300-GHz 7 × 7 detector array based on a 65-nm Si CMOS technology has been developed and transmission imaging was performed using the detector array without any optical elements. The detector array consists of a 7-by-7 arrangement of 49 pixels in a full-chip size of 4 mm × 4 mm. The unit pixel is composed of a single direct detector with a differential common-gate configuration and a differential patch antenna. The fabricated CMOS detector array chip was mounted on a planar package for electrical characterization and imaging. The measured responsivity and the noise equivalent power (NEP) showed best values of 3599 V/W and 12.46 pW/Hz1/2, respectively, at 303 GHz. Various THz imaging experiments were carried out based on the packaged CMOS detector array with a setup that does not require optical elements or the raster scan of the target object.

A design procedure is proposed for triple-push ring oscillators, and an oscillator employing a coupled line-matching network is developed following the procedure. This stepwise procedure, which uses power-dependent Z-parameters of transistors, is applied to the design of each amplifier stage constituting a ring oscillator based on its steady-state oscillation condition. It is verified with both L-section and T-section topologies assumed for the load of the amplifier stages of a given triple-push ring oscillator, and the differences between the two topologies are compared. Based on the procedure, a 270 GHz triple-push ring oscillator that employs coupled lines for matching networks has been developed in a 65-nm complementary metal-oxide-semiconductor (CMOS) process. The circuit benefits from the advantages of coupled lines such as compact area and simplified layout. The fabricated oscillator exhibits a measured oscillation frequency of around 270 GHz and output power of –10.9 dBm, with phase noise of –96 dBc/Hz at 10 MHz offset.

A single-pole single-throw (SPST) switch operating around 300 GHz has been developed in this letter based on a 65-nm CMOS technology. The SPST switch adopts a novel coupled-line topology, in which MOSFETs are employed as a variable impedance component at the through- and coupled-ports to achieve a large isolation. Over the measured frequency band of 220–320 GHz, a minimum insertion loss of 3.9 dB (at 303 GHz) and a maximum isolation of 66 dB (at 250 GHz) were obtained. This achieved peak isolation is the largest value obtained so far beyond 100 GHz with a CMOS switch. An isolation larger than 39 dB was maintained for the entire frequency band measured. The measured return loss showed a maximum value of 29 dB at 312 GHz.

A 24-48 GHz wideband frequency tripler has been developed based on a 130-nm SiGe BiCMOS technology. The 2-stage tripler employs a compact wideband matching technique for the inter-stage and output matching, leading to a measured 3-dB bandwidth of 24 GHz (24-48 GHz), or 67 % with reference to the center frequency. The obtained peak conversion gain is -2.5 dB at 27 GHz with a -5-dBm input power, and the saturated output power is -0.87 dBm at 33 GHz with a 4.8-dBm input power. The tripler consumes 6.8 mW. The circuit occupies 460 × 300 μm2, excluding probing pads.

In this chapter, a review on various electrical methods to generate THz signals and related techniques will be presented. Traditional THz sources based on vacuum devices and diodes will be first discussed, followed by oscillator circuits with transistor technology operating at the THz band. Discussed next will be two effective ways to boost the oscillation frequency of THz signal sources, the n-push technique and frequency multiplication. This will be followed by the description of two widely used power-boosting techniques, amplification and power combining. Lastly, beamforming, which is a special case of power-combining technique that additionally enables the steering of beams with improved directivity, will be treated as a separate topic.

This chapter will provide an overview of various methods of detecting electromagnetic waves, with a focus on THz wave detection. The detection can be made exploiting several different mechanisms. First, various thermal effects caused by THz radiation can be used for detection. Called thermal detectors, this type of detectors include bolometers, pyroelectric detectors, thermopiles, and Golay cells. Second, the rectification of AC signals, often after conversion from electromagnetic waves by antenna, into DC voltage can be used for detection, as in the case for the square-law detectors. Third, there is the heterodyne technique, in which signal is down-converted by a mixer and then detected with a conventional low-frequency AC signal detector. This relaxes the operation frequency requirement for the detector at the back-end of the receiver chain. Historically, the first technique has been widely deployed over infrared detection, while the other two techniques prevailed over the microwave and lower frequency detection. THz wave detection, therefore, can be realized based on these approaches with a proper extension of the frequency range, downward or upward. Yet another detection method that has been dominating the optical band is detection with photodetectors. However, this method does not fit for THz detection since the photon energy corresponding to the THz spectrum is too small (a few meV) for practical detection, and hence will not be treated in this chapter. The rectification and heterodyne techniques can be implemented by either diodes or transistor circuits and will be discussed separately after describing the thermal detector in this chapter. Array detectors will be treated as an additional section.

It was mentioned earlier in this book that there can be two approaches to the THz research, one upward approach from the electronics and the other downward from the optics. While the purpose of this book is to cover the topics relevant to the electronic approach, it is true that the THz research has long been dominated by the approach based on the optics, which has inevitably influenced THz electronics. Therefore, it is desired for the intended readers of this book, who are expected to have their backgrounds in the electronics, to be aware of the basic concepts commonly shared in the optics-driven THz territory. It will promote the interaction with the researchers from the other side of the THz spectrum with a lowered “language barrier” and also help to import the ideas on that side for a productive blending. When those with electronics background attempt to learn the basics of optics-based THz, however, they are often overwhelmed by the amount and details carried by the references and books targeted at the optics majors. This chapter is prepared to serve as a quick guide to optics-based THz for the electronics-based THz researchers. Those who are seeking for a comprehensive coverage of optics-based THz are referred to other references intended mostly for such purposes [1–3]. In this chapter, two major topics will be discussed, which are, naturally, the THz wave generation and detection. The discussion on generation is divided into two sections, one on the pulse and the other on the continuous wave (CW) generation. The section on detection is described mainly for the pulse detection, as the CW detection is commonly performed by electrical means even in the optical environment.

THz waves generated by signal sources (topic in Chap. 2) will be detected by detectors (topic in Chap. 3) after propagating through a medium. In this chapter, the propagation of THz waves and related topics will be discussed. A popular propagation method in the lower frequency ranges, such as RF and microwave, is through coaxial cables. However, the loss will grow with frequency, and it will reach a point where it become prohibitively large for practical uses. Beyond such a point, various types of waveguides, most notably the metallic rectangular waveguides, will be better suited for the propagation of electromagnetic waves. Nevertheless, they also have their own loss-dominated frequency upper limit. The favorable solution beyond this limit would be the propagation through free space, guided by optical components such as lenses and mirrors. The THz band is situated across the region that prefers the propagation through the waveguides or free space. The two propagation schemes will be the main topics in this chapter and described as individual sections. In the case of propagation through free space, usually in the form of Gaussian beam, a device that converts guided waves (or AC signal) to radiated electromagnetic waves (and vice versa) will be needed. The device, or antenna, will be also treated in this chapter as a separate section with focus on its THz application.

This paper presents a high efficiency gate inductive feedback Colpitts Voltage Controlled Oscillator (VCO) operating near 90 GHz. It includes multi-way transformers at the load, which helps to enhance the DC-to-RF efficiency with boosted impedance and extended voltage swing through drain-buffer and drain-source magnetic couplings. The proposed VCO is fabricated in a 65-nm CMOS process, and showed an oscillation frequency tuning range of 87.5-92.0 GHz (4.5 GHz). Over the frequency range, the output power and the dc power consumption exhibited values of −2.8 −1.7 dBm and 6-22.5 mW, respectively. The peak DC-to-RF efficiency was 9.4 %, and the corresponding phase noise was −107.4 dBc/Hz at 10-MHz offset. The lowest phase noise at the optimum bias condition was −116.7 dB/Hz at 10-MHz offset. The chip size is 400 ⨯ 300 μm2 excluding DC and RF pads.

THz oscillators operating up to 680 GHz have been developed in this work based on a 250-nm InP heterojunction bipolar transistor (HBT) technology. This work shows that the common-base cross-coupled push-push oscillator topology is improved by adopting a coupled-line structure, in which DC blocking capacitors and other transmission lines are replaced by a pair of coupled lines. The coupled lines enable efficient impedance and phase matching with a small area, resulting in improved output power and efficiency. Three types of oscillators with a slight dimensional variation were fabricated. The measured oscillation frequency of the three oscillators are 628 - 682 GHz, 556 - 610 GHz and 509 - 548 GHz, respectively, with bias-based tuning. The maximum output power and DC-to-RF efficiency of oscillators are up to -10 dBm and 0.19 %, respectively. The circuit occupies only around 0.014 mm2 excluding the probing pads.

A 283-GHz fully integrated phase-locked loop (PLL) based on a 65-nm CMOS technology is presented. A triple-push ring voltage-controlled oscillator and a frequency divider chain (/16,384) composed of 2 injection-locked frequency dividers are developed, which are integrated with 12 current-mode logic frequency dividers, a phase frequency detector, a charge pump, and a loop filter. The fabricated PLL showed a locking range of 282.3–283.7 GHz and a phase noise of −53.5 dBc/Hz at 100 kHz (in band) and −78.6 dBc/Hz at 10 MHz (out of band). Total dc power consumption is 114 mW. The chip occupies 920 × 520 μ m2 excluding probing pads.

In this paper, three-dimensional (3-D) terahertz (THz) tomography was demonstrated with a signal source and imagers based on transistor circuits fabricated with standard semiconductor technologies. For the signal source, a 300-GHz oscillator based on InP HBT technology was employed. For detection, two types of imagers operating near 300 GHz were employed, one direct and the other heterodyne, both realized with SiGe HBT technology. With a set of 2-D images taken from different angles, sinograms and tomograms were obtained, which led to a successful reconstruction of 3-D images of the target object based on the filtered back-projection algorithm. A systematic comparison was made for the direct imager and the heterodyne imager, for which the signal input power and the video bandwidth were varied for both imagers. The results revealed that the heterodyne imager shows a better sensitivity than the direct imager. However, a similar dynamic range of around 30 dB was achieved for both imagers because of a saturation observed for the heterodyne imager when the input power exceeds the threshold. The video bandwidth did not affect the image quality significantly for the bandwidth variation over four orders of magnitude for both imagers.

A D-band low-noise amplifier (LNA) has been developed based on a 65-nm CMOS technology, which showed a measured peak gain of 16.1 dB at 134.5 GHz. The noise property of the fabricated amplifier was characterized with two different noise measurement techniques: the cryogenic Y-factor method and the N-times power method. The two methods showed a minimum value of the noise figure of 10.7 dB and 14.7 dB, respectively.