Abstract
This paper reviews the state of emerging transistor technologies capable of terahertz amplification, as well as the state of transistor modeling as required in terahertz electronic circuit research. Commercial terahertz radar sensors of today are being built using bulky and expensive technologies such as Schottky diode detectors and lasers, as well as using some emerging detection methods. Meanwhile, a considerable amount of research effort has recently been invested in process development and modeling of transistor technologies capable of amplifying in the terahertz band. Indium phosphide (InP) transistors have been able to reach maximum oscillation frequency (fmax) values of over 1 THz for around a decade already, while silicon-germanium bipolar complementary metal-oxide semiconductor (BiCMOS) compatible heterojunction bipolar transistors have only recently crossed the fmax = 0.7 THz mark. While it seems that the InP technology could be the ultimate terahertz technology, according to the fmax and related metrics, the BiCMOS technology has the added advantage of lower cost and supporting a wider set of integrated component types. BiCMOS can thus be seen as an enabling factor for re-engineering of complete terahertz radar systems, for the first time fabricated as miniaturized monolithic integrated circuits. Rapid commercial deployment of monolithic terahertz radar chips, furthermore, depends on the accuracy of transistor modeling at these frequencies. Considerations such as fabrication and modeling of passives and antennas, as well as packaging of complete systems, are closely related to the two main contributions of this paper and are also reviewed here. Finally, this paper probes active terahertz circuits that have already been reported and that have the potential to be deployed in a re-engineered terahertz radar sensor system and attempts to predict future directions in re-engineering of monolithic radar sensors.
Highlights
Terahertz waves (THz waves) or submillimeter waves are defined as waves with frequencies in the range between 300 GHz and 3 THz [1], lodged between millimeter waves (30 GHz to 300 GHz) and infrared radiation [2,3]
The THz band is suitable for ultra-fast communication; at present telecommunication research is still predominantly focused on the millimeter-wave band and there is no immediate need for exploiting THz communication and research in this area is still in its infancy, at least for the purposes of this review article
It is evident that THz-amplification-capable technologies have been available for over a decade, with III-V compound materials leading the way into applications over 1 THz, but with the disadvantage of high manufacturing costs
Summary
Terahertz waves (THz waves) or submillimeter waves are defined as waves with frequencies in the range between 300 GHz and 3 THz [1], lodged between millimeter waves (30 GHz to 300 GHz) and infrared radiation [2,3]. This means that if only transistors are considered, active circuits can already reach low-THz frequencies without any harmonic generation, and they have excellent potential to extend their regions of operation further to the complete THz band in future This is graphically illustrated, where the THz research gap, in this case defined to overlap with low-THz frequencies (0.1–1 THz), is shown in relation to the frequency spectrum and the fmax capability of available and forecast technologies. Fourth, considerations such as fabrication (and modeling) of passives and antennas [14,39,40] have to be brought in line with technology advances if these are not to hinder the re-engineering of THz sensors with THz-capable transistors.
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