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Highly sensitive plasmonic terahertz detector based on asymmetric FET integrated with sub-wavelength aperture

Kim, Min Jae
Kim, Kyung Rok
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Because of the peculiar characteristics of THz waves, which include penetrability and distinctiveness in feature recognition, Terahertz (THz) technology has great potential. THz imaging technology is unique among THz-frequency range technologies because of its extremely low energy levels, which make it safe for human health. In particular, field-effect transistor (FET)-based THz imaging detectors for real-time THz imaging are presently undergoing extensive research in a multi-pixel array configuration. This is due to the technology's ability to leverage silicon (Si) benefits, which include low cost and high integration density. The plasmonic wave detection technique based on FETs, which is not constrained by the cut-off frequency as in transit mode, has two attractive features: robustness against high THz input power and enhanced responsivity (RV) with rising frequency in the THz range. An analytical device model based on device physics has been built in order to fully comprehend the principles underlying the operation of THz plasmonic detectors. Not constrained by the cut-off frequency as in transit mode, the FET-based plasmonic wave detection mechanism has two attractive features: robustness against high THz input power and enhanced responsivity RV with rising frequency in the THz range. However, because of the diffraction limit, achieving sub-wavelength (λSub) resolution in the THz frequency range—which is characterized by long wavelengths (λ) is intrinsically difficult. A lot of emphasis has recently been paid to near-field approaches, which include both aperture-type and aperture-less (probe-tip) types. These methods remain independent of the THz wavelength; instead, the spatial resolution is dictated by the aperture or tip apex size. In order to improve resolution, a great deal of research has been done on various materials and architectures in aperture-based approaches as well as increasing the field that is transmitted in tip-based techniques. While the overdamped plasma wave drives the FET-based plasmonic detector to function as a power detection mechanism in NR mode, a significant amount of the signal is reflected at the aperture plane as it passes via the aperture. Specifically, for real-time imaging, the transmitted field (E) amplitude diminishes further with increasing pixel count. Addressing the low transmission issue at the aperture-based device level is therefore imperative. In this dissertation, we used 65-nm CMOS technology to examine the effects of aperture location and structural asymmetry on a FET-based plasmonic THz detector with aperture integration. Adding structural asymmetry between the FET's source and drain resulted in a photoresponse (Δu) of 9.3 mV (7-fold) when the aperture was placed close to the gate, compared to a symmetric FET. Furthermore, by creating an asymmetry in feeding the incoming THz wave with the aperture located at the drain, we were able to experimentally demonstrate significantly better detection performance—achieving an 18.5 mV (2-fold) Δu in contrast to the aperture located at the gate.
Ulsan National Institute of Science and Technology


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