**1. Introduction**

led to the development of Si/Si1-xGex

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A MODFET is based on a single or a double heterojunction of two semiconductors with different bandgaps. In an n-channel MODFET, electrons diffuse from a highly doped layer toward a lowly doped layer of a high-mobility material where they are confined by means of the conduction-band discontinuity between the two materials. These electrons form a two-dimensional electron gas (2DEG) conductive channel at the heterointerface. Single-device strained Si/Si0.6Ge0.4 MODFETs with 100-nm T-gates with up to 74-GHz current gain cutoff frequency and 107-GHz maximum oscillation frequency were demonstrated [1].

Si wafers. Therefore, strained-Si MODFETs can lead to single-chip high-performance THz (basic analog building blocks were already demonstrated using Si/SiGe FETs [25]) and guide

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

http://dx.doi.org/10.5772/intechopen.76290

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The devices under study are based on the Si/SiGe system, and the layout of the transistors is shown in **Figure 1(a)**. The epistructure of the MODFETs used in this work is as follows: a thick relaxed linearly graded SiGe virtual substrate is grown over a p-doped conventional Si wafer. The final top Ge molar concentration in the virtual substrate was 0.3. The structure has an undoped 12-nm tensile strained Si channel, sandwiched between two n-doped Si0.70Ge0.30 relaxed supply layers (reddish layers) to generate a high density of electrons in the strained-Si quantum well [26, 27]. The highlighted bluish layer marks out the strained-Si quantum well. Pt/Au was evaporated to fabricate the Schottky gate that was not symmetrically placed between the source and the drain. A more detailed description of the transistor fabrication

The material system Si/SiGe allows the creation of a thin layer of strained silicon under tetragonal (biaxial tensile) strain due to the different values of the Si and the SiGe lattice constants. Tetragonal strain has the effect of lifting the sixfold degeneracy of the conduction band in

**Figure 1.** (a) Epistructure of the Si/SiGe MODFETs showing the vertical layout of the transistors. (b) Conduction and

valence band profiles and the Fermi level under the gate in equilibrium [26].

to future compact, low-cost, high-speed, and high-precision THz detectors.

**2. Device description and TCAD simulations**

**2.1. Strained-silicon MODFET**

can be found in [26].

The terahertz (THz) region (~0.1–10 THz; ~0.03–3 mm; ~3–300 cm−1) lies in the gap between the microwaves and infrared regions of the electromagnetic (EM) spectrum. THz radiations have extraordinary properties; it is a non-ionizing radiation and it is capable of penetrating through many non-conductive materials [2]. THz radiations have shown a great potential in a huge range of THz applications including astronomy [3, 4], spectroscopy (rotational, vibrational, and translational modes in the THz range are specific to a particular substance allowing to obtain a THz fingerprint) [5–7], thickness measurement of multilayer objects [8], communications with a bandwidth significantly higher than those based on microwaves [9], nondestructive inspection based on both imaging of concealed objects and spectroscopy [10], metrology [11], quality control [12], and so on. THz rays (T-rays) permit imaging with a diffractionlimited resolution similar to that of the human eye [13], and, since common optically opaque packaging materials are transparent to T-rays, the inspection of concealed objects is possible.

During the last decade, new promising 2D materials have recently attracted interest to develop room-temperature solid-state THz sensors [14]. However, so far, only devices based on III–V materials [15] and silicon [16] have proved experimentally their potential to build low-cost, compact, scalable, and reliable systems; accordingly, the extension of the frequency range of these devices generates a great interest in THz detection.

Solid-state devices are rising as one of the most promising ways to obtain THz detection and emission at room temperature. Dyakonov and Shur proposed in [17–19] the use of field-effect transistors (FETs) as detectors, multipliers, and mixers in the THz range using the oscillations of the plasma waves in the channel. Nonlinear properties of the two-dimensional plasma permit the detection of the THz radiation. Plasma wave-based detectors can directly convert the incoming EM radiation into measurable voltage or current. They demonstrated that a FET under excitation by THz radiation generates a DC drain-to-source voltage when the drain is open and, therefore, operating in the photovoltaic-mode detection. The value of this voltage can be modulated by the gate-to-source bias voltage, as the gate bias controls the plasma density and therefore the carrier concentration in the FET channel. The increasing availability of continuous-wave compact sources based on solid-state oscillators in the millimeter-wave range raises the interest on the development of direct detectors [20].

This chapter presents a study of room-temperature terahertz detection using strained-silicon modulation field-effect transistors with three different gate lengths. Detection of THz radiation [21, 22] and imaging using Si/SiGe transistors have been previously demonstrated [23, 24]. A main distinct interest of the high-mobility n-type FETs based on the Si/SiGe system is that, unlike the ones based on III–V plasmon detectors, it will be easy to integrate MODFET THz detectors with mainstream Si technology circuits, since both are fabricated on conventional Si wafers. Therefore, strained-Si MODFETs can lead to single-chip high-performance THz (basic analog building blocks were already demonstrated using Si/SiGe FETs [25]) and guide to future compact, low-cost, high-speed, and high-precision THz detectors.
