Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.

https://iopscience.iop.org/article/10.1088/0022-3727/47/37/374001/pdf

A review of terahertz sources

We report on experiments on photoresponse to sub-THz (120GHz) radiation of Si field-effect transistors (FETs) with nanometer and submicron gate lengths at 300K. 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.

Plasma wave detection of sub-terahertz and terahertz radiation by silicon field-effect transistors

We review different techniques for modulation of the electromagnetic properties of terahertz (THz) waves. We discuss various approaches for electronic, optical, thermal and nonlinear modulation in distinct material systems such as semiconductors, graphene, pho- tonic crystals and metamaterials. The modulators are classified and compared with respect to modulation speed, modulation depth and categorized by the physical quantity they control as e.g. amplitude, phase, spectrum, spatial and temporal properties of the THz wave. Based on the review paper, the reader should obtain guidelines for the proper choice of a specific modulation technique in view of the targeted application.

THz Wave Modulators: A Brief Review on Different Modulation Techniques

Resonant 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.

Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications

Resonant 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 THz) 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 instability).

Terahertz emission by plasma waves in 60 nm gate high electron mobility transistors

By using a Monte Carlo simulator, the static and dynamic characteristics of 50-nm-gate AlInAs-GaInAs /spl delta/-doped high-electron mobility transistors (HEMTs) are investigated. The Monte Carlo model includes some important effects that are indispensable when trying to reproduce the real behavior of the devices, such as degeneracy, presence of surface charges, T-shape of the gate, presence of dielectrics, and contact resistances. Among the large quantity of design parameters that enter the fabrication of the devices, we have studied the influence on their performance of two important factors: the doping level of the /spl delta/-doped layer, and the width of the devices. We have confirmed that the value of the /spl delta/-doping must be increased to avoid the reduction of the drain current due to the depletion of the channel by the surface potential. However, a higher /spl delta/-doping has the drawback that the frequency performance of the HEMTs is deteriorated, and its value must be carefully chosen depending on the system requirements in terms of delivered power and frequency of operation. The reduction of the device width has been also checked to improve the cutoff frequencies of the HEMTs, with a lower limit imposed by the degradation provoked by the offset extrinsic capacitances.

/pdf/design-optimization-of-alinas-gainas-hemts-for-high-479raypsd3.pdf

Design optimization of AlInAs-GaInAs HEMTs for high-frequency applications

In order to optimize the low-noise performance of 50-nm-gate AlInAs-GalnAs high-electron mobility transistors (HEMTs), by using an ensemble Monte Carlo simulation we study the influence of three important technological parameters on their noise level: the doping of the /spl delta/-doped layer, the width of the devices and the length of the recess. The noise behavior of the devices is firstly analyzed in terms of the physics-based P, R, and C parameters, and then characterized from a practical (circuit oriented) point of view through their four noise parameters: minimum noise figure, F/sub min/, noise resistance, R/sub n/, and complex input admittance, Y/sub opt/ (or reflection coefficient, /spl Gamma//sub opt/). We have observed an enhancement of the noise when the /spl delta/-doping or the device width are increased (a deterioration parallel to that of f/sub max/). Thus, the optimum noise operation is obtained for the lowest possible values of the /spl delta/-doping and device width. However, for small width the effect of the offset parasitic capacitances makes F/sub min/ increase, thus, imposing a limit for the reduction of the noise. Moreover, the increase of R/sub n/ for small W makes the noise tuning condition critical to reach the optimum low-noise operation. We have also confirmed that when shortening the recess length from 100 to 20 nm at each side of the gate F/sub min/ is reduced, with a slight deterioration of f/sub max/, while the static characteristics are not modified.

Design optimization of AlInAs-GalnAs HEMTs for low-noise applications

Standard layer structure InAlAs/InGaAs/EnP designed for 100 nanometer gate length High Electron Mobility Transistors (HEMTs) become inadequate, if we reduce the gate length under 100 nm. An InAlAs/InGaAs/InP layer structure optimized for 50 nanometer gate length HEMTs has been realized. DC and microwave characteristics are reported on HEMTs realized on a standard layer and an optimized layer, with similar gate length. Comparable cutoff frequencies f/sub T/ are obtained for both devices. The main result is a large improvement of maximum oscillation frequency f/sub max/, which is 260 GHz and 470 GHz for respectively the standard and the optimized devices. This behavior is attributed to the reduction of short channel effects.

T. Parenty

Papers

Terahertz emission by plasma waves in 60 nm gate high electron mobility transistors

Design optimization of AlInAs-GaInAs HEMTs for high-frequency applications

Design optimization of AlInAs-GalnAs HEMTs for low-noise applications

Design and realization of sub 100 nm gate length HEMTs

Design and realization of sub-100 nm gate length HEMTs