The detection of subterahertz (200 GHz) radiation by silicon-oninsulator MOSFETs with submicron gate lengths in the temperature range from $8 to 350 K is reported. The photoresponse measured against gate voltage exhibited a maximum near the threshold voltage with the amplitude decreasing with decreasing temperature. The photoresponse reached the maximum at the gate bias close to the threshold voltage and increased with an increase of the drain-to-source current. This behaviour agrees with the

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... chanism linking the photoresponse to the excitation of the overdamped plasma waves in the transistor channel. The observed effect could be used for nondestructive, contactless testing of silicon very large integrated circuits in situ. Detection of terahertz radiation has potential applications for sensing and characterisation of chemical and biological systems, detection of weapons and explosives, and remote testing. Other applications may span virtually all fields of science, including physics, chemistry, and biology, and will affect electronics, photonics, and imaging technologies. The THz detector is a key component of this technology. Terahertz broadband detectors include bolometers [1] pyroelectric detectors, Schottky diodes [2], and photoconductive detectors [3]. Selective and tunable detectors have an advantage because they require no gratings, moving mirrors, or other elements to analyse the spectrum of incoming radiation. Dyakonov and Shur proposed to utilise the plasma waves in high density electron fluid in an FET channel as voltage tunable nonresonant broadband and resonant detection of THz radiation [4, 5]. Recently, detection of terahertz radiation by plasma waves was demonstrated in commercial AlGaAs=GaAs FETs [6], a double quantum well FET with a periodic grating gate [7], AlGaN=GaN HFETs [8] and Si MOSFETs [9]. In this Letter, we present experimental evidence of the THz detection by silicon-on-insulator (SOI) MOSFETs in the temperature range from 8 to 350 K under different drain bias conditions. An FET, biased by the gate-to-source voltage and subjected to terahertz or subterahertz radiation, can develop a constant drainto-source voltage, which has a resonant dependence on the radiation frequency f 0 ¼ o 0 =2p with maxima at the plasma oscillation frequencies. The width of the resonance curve is determined by the inverse time of the electron momentum relaxation 1=t. The dimensionless parameter, which governs the physics of the problem, is o 0 t. In the regime such that o 0 t ) 1, the FET operates as a resonant detector. When o 0 t ( 1, the plasma oscillations are overdamped, and the FET response is a smooth function of o as well as of the gate voltage (nonresonant broadband detection). According to [8] , the photoresponse in nonresonant regime against gate voltage is given by: