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-of frequency. This work is an overview of some recent results concerning the THz detection by Si MOS transistors with back-gate, low temperatures operation, and circular polarization studies of nanometer scale field effect transistors for the detection of terahertz radiation. Also first results on graphene transistors are discussed.

We use two antenna model to develop a theory of the recently observed helicity-sensitive detection of terahertz radiation by FETs. The effect is due to the mixing of the ac signals produced in the channel by the two antennas. We obtain the helicity-dependent part of the photoresponse and its dependence on the antenna impedance, gate length, and gate voltage. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We have measured the Terahertz photoresponse of three types of FETs as a function of the temperature, to study the change of the dominant current mechanism. For all of them we have compared the photoresponse with its DC transfer characteristics, which is directly proportional to the conductivity.

L'invention concerne un dispositif (1) de mesure de l'état de polarisation d'une onde incidente de fréquence de 10 GHz à 30 THz , comportant un transistor à effet de champ (2), une antenne (3) de réception. Suivant l'invention, les parties (31, 33) d'antenne détectent une composante de polarisation de l'onde, colinéaire à une direction (X) provoquant dans le transistor (2) une tension alternative (Us) de détection, les parties (32, 33) détectent une composante de polarisation de l'onde, colinéaire à une direction (Y) provoquant l'apparition dans le transistor (2) d'une tension alternative (Ud) de détection, le transistor (2) étant agencé pour générer, comme signal électrique (ΔU) de détection entre la borne (21) de source et la borne (22) de drain, une tension continue (ΔU) de détection dont une partie est déterminée par l'état de polarisation elliptique de l'onde par interférence dans le transistor (2) entre les tensions alternatives (Us, Ud).

Terahertz light helicity sensitive photoresponse in GaAs/AlGaAs high electron mobility transistors. The helicity dependent detection mechanism is interpreted as an interference of plasma oscillations in the channel of the field-effect-transistors (generalized Dyakonov-Shur model). The observed helicity dependent photoresponse is by several orders of magnitude higher than any earlier reported one. Also, linear polarization sensitive photoresponse was registered by the same transistors. The results provide the basis for a new sensitive, all-electric, room-temperature, and fast (better than 1 ns) characterisation of all polarization parameters (Stokes parameters) of terahertz radiation. It paves the way towards terahertz ellipsometry and polarization sensitive imaging based on plasma effects in field-effect-transistors.

Temperature dependence of THz detection by field effect transistors was investigated in a wide range of temperatures from 275K down to 5 K. The important increase of the photoresponse following 1/T functional dependence was observed when cooling from room temperature down to 30K. At the temperatures below similar to 30 K, the THz response saturated and stayed temperature independent. Similar behavior was observed for GaAs, GaN, and Si based field effect transistors. The high temperature data were successfully interpreted using recent theory of overdamped plasma excitation in field effect transistors. The low temperature saturation of the photoresponse was tentatively explained by the change of the transport regime from diffusive to ballistic or traps governed one. Our results clearly show that THz detectors based on field effect transistors may improve their responsivity with lowering temperature but in the lowest temperatures (below similar to 30 K) further improvement is hindered by the physics of the electron transport itself.

AlGaN/GaN based FETs have great potential as sensitive and fast operating detectors because of their material advantages such as high breakdown voltage, high electron mobility, and high saturation velocity. These advantages could be exploited for resonant and non-resonant terahertz detection. We have designed, fabricated, and characterized AlGaN/GaN based FETs as single pixel terahertz detectors. This work focuses on non-resonant detection and imaging using GaN field plate FETs. To evaluate their performances as terahertz detectors, we have measured the responsivity as a function of gate voltage, the azimuthal angle between the terahertz electric field, the source-to-drain direction, and the temperature. A simple analytical model of the response is developed. It is based on plasma density perturbation in the transistor channel by the incoming terahertz radiation. The model shows how the non-resonant detection signal is related to static (dc) transistor characteristics and it fully describes the experimental results on the non-resonant sub-terahertz detection by the AlGaN/GaN based FETs. The imaging performances are evaluated by scanning objects in transmission mode and an example of application of terahertz imaging as new non-destructive technique for the quality control of materials is given. Results indicate that these FETs can be considered as promising devices for terahertz detection and imaging applications.