Resonant frequencies of the two-dimensional plasma in field effect transistors (FETs) increase with the reduction of the channel dimensions and can reach the terahertz (THz) range for micrometer and sub-micrometer channel lengths. Non linearity of the gated electron gas in the transistor channel can be used for the detection of THz radiation. The possibility of tuneable narrow band detection in sub-THz and THz range, related to plasma resonances, has been demonstrated for nanometre gate length transistors at cryogenic temperatures. At room temperatures the plasma oscillations are usually strongly damped, but field effect transistors can still operate as an efficient broadband detectors in the THz range. We present an overview of experimental results on THz detection by field effect transistors made of III-V and Si materials, The material issue is discussed and first room applications of FETs for imaging at frequencies above 1 THz are demonstrated.

The possibility of using commercial classical near-infrared (similar to 30 THz) focal-plane micro-bolometer arrays in the sub-terahertz frequency range is studied. Real-time room-temperature pixilated detector imaging of a 0.5 THz (lambda = 600 mu m) spatial beam pro. le emitted from a backward-wave oscillator is presented. Images, with a signal-to-noise ratio of 5 are demonstrated, leading to an estimate of the noise equivalent power of 11 nW . Hz(-1/2).

Field Effect Transistors for Terahertz Detection

For optical frequencies, there are systems known from the prior art which not only make it possible to represent an object two-dimensionally, but also detect it three-dimensionally, in other words to determine the distance between the object and the detector for individual pixels. In contrast, the object of the present invention is to provide a system and a method for detecting an object three-dimensionally using high-frequency electromagnetic radiation at a frequency in a range of 100 GHz to 10 THz. According to the invention, this object is met by a system for detecting an object three-dimensionally using high-frequency electromagnetic radiation, the system comprising a radiation source for radiating high frequency radiation at a frequency in a range of 100 GHz to 10 THz, a receiver for detecting the high frequency radiation emitted from the radiation source, said receiver comprising an antenna element and a field effect transistor for mixing the high frequency radiation emitted by the radiation source with a local oscillator signal, wherein the field effect transistor comprises a source, a drain and a gate, and comprising a signal source for generating an electric modulation signal, wherein the signal source is connected to the radiation source in such a way that the high frequency radiation can be modulated by the modulation signal, and wherein the signal source is connected to the gate of the field effect transistor in such a way that the sensitivity of the field effect transistor can be modulated using a local oscillator signal coupled at a fixed phase to the modulation signal.

Imaging at 0.6 THz is tested with a commercial GaAs high-electron-mobility transistor (HEMT) operated at room temperature. The results allow the assessment of the potential of future antenna-fitted HEMT arrays for real-time imaging.

The channel of a field effect transistor can act as a resonator for plasma waves propagating in a two-dimensional electron gas. The plasma frequency increases with reduction of the channel length and can reach the terahertz (THz) range for nanometer size transistors. Recent experimental results show these transistors can be potential candidates for a new class of THz detectors and emitters. This work gives an overview of our recent relevant experimental results. We also outline unresolved problems and questions concerning THz detection and emission by nanometer transistors.