Room-Temperature Terahertz Detection and Imaging by Using Strained-Silicon MODFETs

Abstract

This chapter reports on an experimental and theoretical study of Schottky-gated strained-Si modulation-doped field-effect transistors (MODFETs) with different sub-micron gate lengths (100, 250, and 500 nm). Room-temperature detection of terahertz (THz) radiation by the strained-Si MODFETs was performed at two frequencies (0.15 and 0.3 THz). A technology computer-aided design (TCAD) analysis based on a two-dimensional hydrodynamic model (HDM) was used to investigate the transistor response to THz radiation excitation. TCAD simulation was validated through comparison with DC and low-frequency AC measurements. It was found that the photoresponse of the transistors can be improved by applying a constant drain-to-source bias. This enhancement was observed both theoretically and experimentally. The HDM model satisfactorily describes the experimental dependence of the photoresponse on the excitation frequency, the gate bias, and the drain-to-source current bias. The coupling of the incoming THz radiation to the MODFETs was studied at 0.15 and 0.3 THz. Finally, to demonstrate the suitability of strained-Si MODFET for terahertz applications, an image sensor within a pixel-by-pixel terahertz imaging system for the inspection of hidden objects was used.

Numerical Study of the Coupling of Sub-Terahertz Radiation to n-Channel Strained-Silicon MODFETs

“…This does not mean that the gate length does not have an impact on the responsivity of the device. On the contrary, the Dyakonov-Shur theory predicted [9] that the gate length plays an important role in the detector performance in agreement with the previous measurements [21], but not in the coupling of the radiation as the length is considerably smaller than the wavelength of the radiation.…”

“…In this transistor a double heterojunction SiGe/Si/SiGe is used to, on the one hand, establish a tetragonal (biaxial tensile) strain on the internal silicon layer (i.e., the transistor channel) and, on the other hand, create two supply layers in the two unstrained SiGe layers around the channel. The combination of the modulation doping, the strain of the channel layer, the dual-supply definition, and the asymmetrical placement of the gate between source and drain (Figure 1b) ensure an excellent performance of the strained-Si MODFET as a sub-THz detector [19][20][21] that will allow it to compete with detectors based on other technologies. These technologies include, among others, THz detectors based on mainstream CMOS [22] that have achieved responsivities close to 350 V/W at 0.6 THz, plasmonic detectors based on InAlAs/InGaAs/InP high electron mobility transistors (HEMTs) with an asymmetric dual-grating-gate structure exhibiting a responsivity of 22.7 kV/W at 200 GHz in the photovoltaic mode [23,24] at room temperature and forthcoming technologies based on two-dimensional materials [25].…”

“…Figure 1a presents the vertical layout of the MODFETs whose sub-THz response is studied in this work along with the schematic of the overhead front THz illumination. Further details related to this vertical layout and device performance are given in [21]. The main geometrical parameters of the transistor were: the total drain-to-source distance (L sd ) was 2 µm, the gate width (W g ) was 60 µm, the separation between the right edge of the source contact and the left edge of the gate (L gs ) was 1 µm, and the gate length (L g ) was L g = 250 nm.…”

Review on Physics and Modelling of THz Radiation Detection Using Field-effect Transistors

“…This is the first theory for THz detection in FET proposed by Dyaknov and Shur [20]. It relies on the THz radiation's excitation of plasma waves in the FET's channel to produce a DC output [16,27,28]. However, this perception conflicts with the conventional theoretical concept of plasma.…”