With the exponential growth of the data traffic in wireless communication systems, terahertz (THz) frequency band is envisioned as a promising candidate to support ultra-broadband for future beyond fifth generation (5G), bridging the gap between millimeter wave (mmWave) and optical frequency ranges. The purpose of this paper is to provide a comprehensive literature review on the development towards THz communications and presents some key technologies faced in THz wireless communication systems. Firstly, despite the substantial hardware problems that have to be developed in terms of the THz solid state superheterodyne receiver, high speed THz modulators and THz antennas, the practical THz channel model and the efficient THz beamforming are also described to compensate for the severe path attenuation. Moreover, two different kinds of lab-level THz communication systems are introduced minutely, named a solid state THz communication system and a spatial direct modulation THz communication system, respectively. The solid state THz system converts intermediate frequency (IF) modulated signal to THz frequency while the direct modulation THz system allows the high power THz sources to input for approving the relatively long distance communications. Finally, we discuss several potential application scenarios as well as some vital technical challenges that will be encountered in the future THz communications.

A survey on terahertz communications

This paper reviews the rapid advancements being made in the development of high electron mobility transistors (HEMTs) on InP substrates for future sub-millimetre wave (30–300 GHz) and terahertz (300 GHz to 3.5 THz) frequency applications. The InP HEMTs exhibits outstanding 2-DEG properties in InAlAs/InGaAs heterostructure. This paper highlights the rapid growths in the developments of enhancement mode (E-Mode) and depletion mode (D-Mode) InP HEMTs over the last 40 years, the use of InP HEMTs for cryogenic applications, reliability issues and kink effects in InP HEMTs in detail and it also highlights the impact of geometrical dimensions and their scaling on the performance of InP HEMTs. Their uniqueness in terms of low noise, low power dissipation, high gain and high frequency of operation has fuelled the incorporation of InP HEMTs in a wide variety of applications such as high speed wireless and optical communication systems, sub-millimetre wave (S-MMW) and THz receivers and transmitters, radio astronomy and radiometry, flight communications and sensing applications, material spectroscopy, active and passive imaging applications, biomedical instrumentation applications and high speed ICs such as low noise amplifiers (LNAs), multiplier chains, switches, multiplexers and flip-flops.

InP high electron mobility transistors for submillimetre wave and terahertz frequency applications: A review

A critical review of design and fabrication challenges in InP HEMTs for future terahertz frequency applications

This paper reports demonstration of a 0.666 THz (666 GHz) datalink. A peak error-free data rate of 9.5 Gbps is demonstrated over a 590 m range using QPSK modulation with a measured humidity of 18% with more than 40 dB SNR at the output of the receiver. The 0.666 THz link uses InP HEMT frequency converters providing both low noise and power amplification, a traveling wave tube Amplifier, and an advanced modem architecture. This ground-based demonstration indicates that traditional communications approaches can be used well into the submillimeter wave range. Moreover, a space-based crosslink based on this technology should be capable of transmitting Gbps-class data rates at LEO-LEO ranges.

A 666 GHz demonstration crosslink with 9.5 Gbps data rate

In this paper, the DC and RF characteristics of our own InP-based HEMTs were investigated comprehensively with surface traps concentration varying among 0 cm−2, 1 × 1012 cm−2, 3 × 1012 cm−2. Reasonable physical models were used to describe the device performances, including hydrodynamic transport model, density gradient model, recombination models and so on. Moreover, surface Shockley–Read–Hall model and improved Possion's equation were introduced to implement for the impact of surface traps. The DC and RF characteristics have shown compatible trends between computation and experiment. With the increasing surface traps concentration over the gate-recess region, the channel current, transconductance, current gain cut-off frequency (fT) and maximum oscillation frequency (fmax) exhibit apparent decrease trends. The observed obvious changes of the DC and RF properties with surface traps could be accounted for carrier sheet density reduction, which resulted from the surface potential modulated by negatively charged surface traps. Additionally, the charged surface traps could deplete the electrons in channel region, which leads to an increase in channel sheet resistance, and thus, further decreasing the fmax.

Numerical simulation of the impact of surface traps on the performance of InP‐based high electron mobility transistors

In this letter, a 670-GHz receiver using 25-nm InP HEMT technology is presented. The receiver obtains the lowest reported noise figure for a transistor-based receiver operating at this frequency (<10.3 dB), while consuming a total dc power of only 1.8 W. This includes dc power consumption of a ×18 multiplier chain for the local oscillator and linear dc regulators. These results show that InP HEMT technology provides high sensitivity at submillimeter wave frequencies with lowest possible dc power consumption.

A Low-Power 670-GHz InP HEMT Receiver

In this work, a novel 670-GHz integrated direct-detection receiver using 25-nm InP HEMT technology is presented. This is the first demonstration of an integrated direct-detection radiometer architecture at these frequencies. The receiver exhibits a noise figure of 11.4 dB with a total dc power consumption of 0.25 W. The integrated receiver measures only 0.8 cm × 1.3 cm × 4.8 cm (0.3” × 0.5” × 1.9”). These results show that transistor-based direct-detection receivers are a viable technology for submillimeter-wave applications, with low SWaP with no compromise in performance.

Gerry X. B. Mei

Papers

A Low-Power 670-GHz InP HEMT Receiver

A 670 GHz Integrated InP HEMT Direct-Detection Receiver for the Tropospheric Water and Cloud Ice Instrument