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Field-Effect Transistor as a THz Detector for Sub-Wavelength THz Imaging


NSF funded researchers at Rensselaer Polytechnic Institute developed a THz imaging technique employing a plasma wave field-effect transistor as a detector. Field-effect transistors with nano-scale gates detect terahertz radiation at frequencies far above their cutoff frequencies via excitation of resonant or damped plasma waves (which are waves of electron density in 2D electron gas in the transistor channel). A typical plasma wave velocity may exceed the electron saturation velocity by an order of magnitude or more, and the maximum detectable frequency, defined by the transient time of plasma waves under the gate can exceed 10 THz (see [1-3] and references wherein).

Students of Prof. Shur, supported by the NSF through the Integrated Graduate Education and Research Traineeship (IGERT), have demonstrated that micrometer scale and even nanometer scale resolution imaging can be achieved with the field effect transistor operating in a plasma wave detector mode by varying drain and gate biases. Figure 1 shows the transistor sensitivity pattern. This image was recorded as a variation of the transistor response induced by the THz laser radiation, and measured between drain and source contacts, versus the displacement of the transistor in perpendicular plane in respect to the focused laser beam.

The typical images exhibit two different positions of the response maxima with the different signs. Analysis of the spatial pattern revealed that the transistor response had contributions of different signs from two spatially-separated, effective THz sources, which were attributed to radiation-induced signals between source-and-gate and gate-and-drain electrodes of the device. The strength of these sources varies depending on beam polarization, orientation, position, and even transistor biasing conditions. Variation in strength of these two competing contributions causes modification of the transistor responsivity pattern. The most interesting result is the shift in the position of the maxima with the drain bias when the transistor is in the saturation region. In this regime, the only dramatic change is in the electric field distribution at the drain side of the channel, where the peak field and the width of the high field region increase with the drain bias. The computed and estimated change in the width of this high field region is in the nanometer range, whereas the measured relative shift in the position of the response maximum is in the micrometer range (see Fig. 2). The dependence in Fig. 2 could be used as a calibration curve for determining the positions of the peak response as function of bias of FETs of different design.

One interesting application might be the designing of the field termination plate of the powerful transistors. In this application, the new characterization technique should allow one to determine the width of the high field region with and without the field-plate and its dependence on the drain bias. Another application is in studying the effect of illumination, magnetic field and temperature on the electric field distribution in the channel of FETs. Still another application is the investigation of space variation of THz response of bio- and chemical compounds placed in proximity of a field effect transistor or matrix of FETs. While a number of researchers have reported on sub wavelength imaging with THz radiation using sharp needles [4,5] subwavelength diaphragms 6, or optically-induced diaphragms 7, in the technique described here, the sub-wavelength resolution is reached because of the nano-scale feature sizes of the detector.

1. M. Dyakonov and M.S. Shur, IEEE Transaction on Electron Devices 43, 380 (1996). 2. W.J. Stillman and M.S. Shur, Journal of Nanoelectronics and Optoelectronics 2, 209 (2007). 3. D. Veksler, F. Teppe, A. P. Dmitriev, V. Yu. Kachorovskii, W. Knap, and M. S. Shur, Phys. Rev. B 73, 125328 (2006). 4. R. Merz, F. Keilmann, R.J. Haug, and K. Ploog: Phys. Rev. Lett. 70, 651 (1993). 5. H.-T. Chen et al., “Terahertz imaging with nanometer resolution”, Appl. Phys. Lett. 83, 3009 (2003). 6. O. Mitrofanov, I. Brener, R. Harel, J. D. Wynn, L. N. Pfeiffer, K. W. West, and J. Federici, Appl. Phys. Lett. 77, 3496 (2000). 7. Q. Chen, Z. Jiang, G. X. Xu, and X.-C. Zhang, Optics Letters 25, n.15, 1122 (2000).

Address Goals

The terahertz (THz) frequency band of the electromagnetic radiation is important for various spectroscopic applications, since unique vibration, rotational, and translational responses of materials belong to the THz band. [M. K. Choi, A. Bettermann, and D. W. van der Wiede, “Potential for detection of explosive and biological hazards with electronic terahertz systems,” Phil. Trans. R. Soc. Lond. A, 362, pp. 337-349 (2004).; F. De Lucia, “Spectroscopy in the terahertz spectral region,” in Sensing With Terahertz Radiation, D. Mittleman, Ed. Springer Verlag, Berlin, pp. 39-115 (2003)].

Many THz applications, such as medical imaging, require higher resolution than the diffraction limit, and sub-wavelength imaging techniques are required to achieve better resolution. Such techniques employ sharp needles concentrating the THz field near the tip, subwavelength diaphragms, or optically-induced diaphragms. In this highlight we show, that micrometer scale and even nanometer scale resolution imaging can be achieved with the field effect transistor operating in a plasma wave detector mode by changing drain and gate biases. This might become a crucial point for chemical and biological THz applications that require superfine spatial resolution along with spectroscopic information.