22 pages, 12 figures, review paperInternational audienceResonant frequencies of the two-dimensional plasma in FETs increase with the reduction of the channel dimensions and can reach the THz range for sub-micron gate lengths. Nonlinear properties of the electron plasma in the transistor channel can be used for the detection and mixing of THz frequencies. At cryogenic temperatures resonant and gate voltage tunable detection related to plasma waves resonances, is observed. At room temperature, when plasma oscillations are overdamped, the FET can operate as an efficient broadband THz detector. We present the main theoretical and experimental results on THz detection by FETs in the context of their possible application for THz imaging

We report on the resonant, voltage tunable emission of terahertz radiation ͑0.4 -1.0 THz͒ from a gated two-dimensional electron gas in a 60 nm InGaAs high electron mobility transistor. The emission is interpreted as resulting from a current driven plasma instability leading to oscillations in the transistor channel ͑Dyakonov-Shur instability͒.Plasma waves in a gated two-dimensional electron gas have a linear dispersion law, similar to that of sound waves. The transistor channel acts as a resonator cavity for plasma waves that can reach THz frequencies for a sufficiently short ͑nanometer-sized͒ field effect transistor. 1 As was predicted in Ref. 2, when a current flows through a field effect transistor, the steady state can become unstable against the generation of plasma waves ͑Dyakonov-Shur instability͒ leading to the emission of an electromagnetic radiation at plasma wave frequencies. The emission is predicted to have thresholdlike behavior. It is expected to appear abruptly after the device current exceeds a certain threshold value for which the increment of the plasma wave amplitude exceeds losses related to electron collisions with impurities and/or lattice vibrations.The excitation of plasma waves in a field effect transistor channel can be also used for the detection of terahertz radiation. 3 Recent reports demonstrated a resonant 4 detection in GaAs-based high electron mobility transistors ͑HEMTs͒ and in gated double quantum well heterostructures. 5 This is the first report of resonant THz emission by plasma generation. The terahertz emission ͑0.4 -1.0 THz͒ was obtained by using an InGaAs HEMT with a 60-nm-long gate. We show that the results can be interpreted assuming that the emission is caused by the current driven plasma instability leading to terahertz oscillations in the channel through Dyakonov-Shur instability.Lattice-matched InGaAs/AlInAs HEMTs grown by molecular beam epitaxy on an InP substrate were used in this study. The active layers consisted of a 200 nm In 0.52 Al 0.48 As buffer, a 20 nm In 0.53 Ga 0.47 As channel, a 5-nm-thick undoped In 0.52 Al 0.48 As spacer, a silicon planar doping layer of 5ϫ10 12 cm Ϫ2 , a 12-nm-thick In 0.52 Al 0.48 As barrier layer, and, finally, a 10-nm-silicon-doped In 0.53 Ga 0.47 As cap layer. Details of the technological process are given elsewhere. 6 The gate length was 60 nm, and the drain-source separation was 1.3 m. An InP-based HEMT was chosen for its high InGaAs channel mobility and high sheet carrier density.Output and transfer characteristics are shown in Fig. 1. The low field, linear output region is marked by the dotted line. The deviation of the I d (U sd ) curve from linear behavior indicates the beginning of the saturation region. The arrow indicates the emission threshold voltage, U sd ϳ200 mV at I d ϳ4.5 mA. The horizontal dashed line shows the level of the current saturation (I d ϳ4.8 mA). The I d (U sd ) characteristic shows an unstable behavior for U sd higher than 300 mV. This well-known phenomenon is related to a self-excitation a͒ Also at

We report on experiments on photoresponse to sub-THz ͑120 GHz͒ radiation of Si field-effect transistors (FETs) with nanometer and submicron gate lengths at 300 K. The observed photoresponse is in agreement with predictions of the Dyakonov-Shur plasma wave detection theory. This is experimental evidence of the plasma wave detection by silicon FETs. The plasma wave parameters deduced from the experiments allow us to predict the nonresonant and resonant detection in THz range by nanometer size silicon devices-operating at room temperature.