Field Effect Transistors for Terahertz Detectors

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Electromagnetic waves in the terahertz frequency range can easily penetrate packaging materials and render image details in high resolution, and can also detect the chemical fingerprints of pharmaceutical drugs, biological weapons, or illegal drugs or explosives. However, most existing terahertz systems involve bulky and expensive laser setups that sometimes require exceptionally low temperatures. The potential of terahertz imaging and scanning has gone untapped because of the lack of compact, low-cost technology that can operate in the frequency range.

Nanometer size field effect transistors can operate as efficient resonant or broadband terahertz detectors, mixers, phase shifters and frequency multipliers at frequencies far beyond their fundamental cut-off frequency. The real large-scale interest in using FETs as THz detectors started around 2004 after the first experimental demonstration of sub-THz and THz detection in silicon-CMOS FETs Knap W et al 2004.

Prof. Michael Shur’s group reported an enhanced room-temperature detection of terahertz radiation by several connected field-effect transistors. For this enhanced nonresonant detection, they designed, fabricated, and tested plasmonic structures consisting of multiple InGaAs/GaAs pseudomorphic high electron-mobility transistors connected in series shown in the photo. Shur’s work with plasma wave excitation in submicron field effect transistors (FET) and related device structures should allow the development of a new generation of solid-state terahertz (THz) tunable devices. Tauk R et al 2006 has shown that Si-CMOS FETs can reach a noise equivalent power competitive with the best conventional room temperature THz detectors. Both pioneering works have clearly stated the importance of Si-CMOS FETs, which present the advantages of room temperature operation, very fast response times, easy on-chip integration with read-out electronics, leading to straight-forward array fabrication. Recent studies demonstrate the detector characteristics, responsivity and noise equivalent power, within the same range as Schottky barrier diodes. Other existing THz detectors, such as bolometers, Golay cells, and pyroelectric detectors, require specialized and less common fabrication technologies, which do not allow monolithic integration.

Mona Jarrah work and Ullrich Pfeiffer from the University of Wuppertal, in Germany, have both built antenna and lenses for THz radiation and very efficient transistor THz detectors. In letter entitled “Terahertz response of field effect transistor operating in the saturation regime” was first to present present the enhancement of THz detection by operating transistor in saturation regime and achieve detected signal amplification of small THz signal beyond the cut-off frequency.  In the paper, the broadband terahertz response of InGaAs/GaAs high electron mobility transistors operating at 1.63 THz and room temperature deep in the saturation regime is reported.

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