International Journal of High Speed Electronics and Systems

THz technology applications, sensors and sources are briefly reviewed. Emphasis is placed on the less familiar components, instruments or subsystems. Science drivers, some historic background and future trends are discussed.

The terahertz frequency range of the electromagnetic spectrum holds great promise for many applications including sensing, imaging, and communications. However, the availability of solid-state sources with reasonable power levels is well recognized as one of the major obstacles for systems applications in this frequency range. Here, the state of the art of active two-terminal devices is reviewed and high-lighted with some exemplary experimental results. The potential, capabilities, and limitations of two-terminal devices regarding power generation at terahertz frequencies are discussed. Furthermore, a new device is described that has the potential of circumventing some of the limitations of existing devices.

Harmonic generation based on frequency multipliers has proven to be the most successful and widely used solid-state technology for generating power at submillimeter wavelengths. Over the last several years, the development of new device technologies, implementation of innovative circuits, and application of advanced integrated-circuit processing techniques to frequency multiplier design have resulted in unprecedented levels of performance throughout the submillimeter-wave frequency band. This paper reviews the technological innovations, device options, circuit architectures, and fabrication technologies that have made harmonic generation such a successful approach to source development in the submillimeter spectrum.

Transistor bandwidths are approaching terahertz frequencies. Paramount to high speed transistor operation is submicron device scaling. High bandwidths are obtained with heterojunction bipolar transistors by thinning the base and collector layers, increasing emitter current density, decreasing emitter contact resistivity, and reducing the emitter and collector junction widths. In mesa HBTs, minimum dimensions required for the base contact impose a minimum width for the collector junction, frustrating device scaling. We have fabricated HBTs with narrow collector junctions using a substrate transfer process. HBTs with submicron collector junctions exhibit extremely high f_max and high gains in mm-wave ICs. Transferred-substrate HBTs have obtained record 21 dB unilateral power gain at 100 GHz. Recently-fabricated devices have shown unbounded unilateral power gain from 40-110 GHz, and f_max cannot be extrapolated from measuremente. However, these devices exhibited high power gains at 220 GHz, the frequency limit of presently available microwave network analyzers. Demonstrated amplifier ICs in the technology include reactively tuned amplifiers at 175 GHz, lumped and distributed amplifiers with bandwidths to 85 GHz, and W-band power amplifiers.

This article reviews the fundamental concepts, technology, and recent developments on the topic of THz photomixers made from low-temperature-grown GaAs, ErAs:GaAs, and ErAs:InGaAs. It is designed to be comprehensive and useful both to students and to engineers and scientists new to the THz field.

The prospects and advantages of silicon germanium quantum cascade lasers are discussed, from both physical and technological perspectives. A range of Si/SiGe intersubband laser configurations are discussed, for both edge and surface emission. Recent experimental activity on mid- and far-infrared devices is reviewed, and the value of detailed theoretical tools for heterostructure design is highlighted. Steps towards silicon optoelectronic integration are also considered.

Plasma waves are oscillations of electron density in time and space. In deep submicron field effect transistors plasma wave frequencies lie in the terahertz range and can be tuned by applied gate bias. Since the plasma wave frequency is much larger that the inverse electron transit time in the device, it is easier to reach "ballistic" regimes for plasma waves than for electrons moving with drift velocities. In the ballistic regime, no collisions of electrons with impurities or lattice vibrations occur on a time scale on the order of the plasma oscillation period, and the device channel acts as a resonant cavity for the plasma waves, making possible tunable resonant detection or even emission of the electromagnetic radiation in the terahertz range. We review the theory of plasma waves in field effect transistors; discuss instabilities of these waves in different device structures and their applications for detection and generation of the terahertz radiation.

Terahertz (THz) radiation occupies part of the electromagnetic spectrum between the infrared and microwave bands. Until recently, technology at THz frequencies was under-developed compared to the rest of the electromagnetic spectrum, leaving a gap between millimeter waves and the far-infrared (FIR). In the past decade, interest in the THz gap has been increased by the development of ultrafast laser-based T-ray systems and their demonstration of diffraction-limited spatial resolution, picosecond temporal resolution, DC-THz spectral bandwidth and signal-to-noise ratios above 10⁴.

This chapter reviews the development, the state of the art and the applications of T-ray spectrometers. Continuous-wave (CW) THz-frequency sources and detectors are briefly introduced in comparison to ultrafast pulsed THz systems. An emphasis is placed on experimental applications of T-rays to sensing and imaging, with a view to the continuing advance of technologies and applications in the THz band.

Recent advances in the technique of terahertz time-domain spectroscopy have led to the development of the first fiber-coupled room-temperature broadband terahertz sources and detectors. The fiber coupling permits the repositioning of the emitter and receiver antennas without loss of temporal calibration or alignment, thus enabling multistatic imaging. We describe a new imaging method which exploits this new capability. This technique emulates the data collection and image processing procedures developed for geophysical prospecting. We use a migration procedure to solve the inverse problem; this permits us to reconstruct the location, shape, and refractive index of targets. We show examples for both metallic and dielectric model targets, and we perform velocity analysis on dielectric targets to estimate the refractive indices of imaged components. These results broaden the capabilities of terahertz imaging systems, and also demonstrate the viability of the THz system as a test bed for the exploration of new seismic processing methods.