**1. Introduction**

The terahertz region of the electromagnetic spectrum roughly extends from 0.1 to 10 THz, corresponding to wavelengths from 3 mm to 30 μm. Terahertz (THz) waves interact with a plethora of materials including solid state, chemical, and biological systems. Consequently, THz waves offer numerous applications including material characterization, imaging, wireless communication, and so on [1–4]. Such real-world applications of THz technology necessitate versatile adaptive optical components such as modulators, filters, lens, switches, waveguide,

© 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. © 2018 The Author(s). Licensee IntechOpen. 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.

polarizer, and so on. Among all these desired THz components and devices, modulators stand at on the focus of current interest [5]. Modulators can be used to control the amplitude, phase, polarization state, spatial propagation direction, pulse shape, and many more characteristic properties of electromagnetic waves and thus act as essential parts of sophisticated THz application systems such as wireless telecommunication or security imaging [6].

time of semiconductors, which for intrinsic or HR silicon is beyond 10 μs, limiting the modulation speed to a maximum of ~100 kHz. Though gallium arsenide (GaAs) has very short carrier lifetime (10–100 ps), it requires high-power pulsed laser excitation up to 1 kW/cm2

achieve similar results as Si [27]. The reason for this is that the carrier lifetime of semiconductor affects the modulation depth and speed in an opposite way. Further research is ongoing

Another method of THz modulation is to thermally tune the electrical conductivity and thus the optical response of semiconductors or metal oxides, especial those materials with insulator-

transition material that exhibits a reversible first-order phase transition from an insulating state to a metallic state above room temperature (~68°C). Associated with this metal-insulator transition (MIT) is a lattice structural transition from the monoclinic to tetragonal, a change of conductivity by 3–5 orders of magnitude and significant changes of the optical properties at all wavelengths. Since the insulated state is transparent while conductive state is opaque to THz wave, the THz transmission can thus be dynamically modified from transparent to reflecting

with resonant element (e.g., metamaterials), have already been used to control and manipulate

Although great progress has been made in optically and thermally driven THz modulators, an all-electronic approach is more interesting and attractive for the real application. It is well known that the carrier concentration in semiconductors can be tuned by electric injection or depletion of charge carriers. It is proved that THz wave can be manipulated by the use of two-dimensional electron gases (2DEGs) in semiconductors [7, 8]. A semiconductor-based field-effect transistor (FET) is a very useful architecture to fabricate effective THz modulators.

Graphene is a quasi-two-dimensional isolated monolayer of carbon atoms that are arranged in a hexagonal lattice. It is well known for its remarkable electron mobility at room tem-

ties were expected to be nearly identical. Graphene holds great promise for various material/ device applications, including solar cells [35], light-emitting diodes (LED), touch panels, and

Graphene is a zero-band-gap semiconductor where conduction and valence bands meet at the Dirac points. The band gap is an extremely important characteristic of the semiconductor for transistor application, which enables the transistor device to turn off and minimizes leakage current at off state. In this circumstance, the gapless band structure of single-layer graphene makes it unsuitable for the direct use of graphene- based field-effect transistors (FET), though it is one of the most widely discussed applications in electronics. In order to overcome the challenges faced in incorporating graphene into microelectronic applications, great efforts have gone into developing three main aspects including developing a synthesis technique to manufacture graphene over wafer-scale areas, forming a high-quality gate dielectric on the surface of graphene, and, most importantly, opening an energy band gap in graphene. This

film. VO2

to overcome this problem [27].

THz wave [15, 30, 32, 33].

metallic phase transition [15, 28–31]. Vanadium dioxide (VO2

modes by controlling the phase transition of the VO2

**2. Graphene and graphene field-effect transistors**

perature, with reported values in excess of 15,000 cm2

smart windows or phonesal [36].

to

119

), for example, is a typical phase

Graphene Field-Effect Transistor for Terahertz Modulation

http://dx.doi.org/10.5772/intechopen.76744

films, separately or integrated

·V−1·s−1 [34]. Hole and electron mobili-

However, unlike the optical or microwave regime where active modulators are well established, the THz frequency regime is still in great demand for efficient, fast, and versatile active light modulators [6]. This is mainly due to the lack of natural materials that have tunable electromagnetic response to THz wave [5]. Nevertheless, materials including semiconductors [7, 8], metamaterials [9–12], superconductors [13, 14], and phase-transition materials [15, 16] have been intensively explored to control and manipulate THz wave with great progress being made in this direction. Modulators can be categorized by the technique or material system, which is employed to modulate the wave, for example, optical, electronic, thermal, and magnetic modulators.

The all-optical approach is an effective and attractive method to fabricate THz modulators, especially those with broadband or spatial operating features. Semiconductors, especially high-resistive (HR) silicon, have proven to be suitable for all-optical modulation of THz wave by converting photons into electrons upon optical illumination [17]. Generally, the pump laser produces a temporary region of high absorption or reflectance on semiconductor. THz wave, co-projected on this area, is thus modulated [18]. As a result, all optical spatial THz modulators, based on a bare silicon wafer, have previously been proposed to realize photodesigned THz devices [19] or reconfigurable quasi-optical THz components [20, 21]. Si is an attractive candidate for optical opponents because it is earth abundant, chemically stable, and has a suitable band gap. Silicon-based THz devices are particularly desirable as they would enable interfacing with existing and emerging Si-based optoelectronics, thereby providing potential low-cost route toward applications. However, a silicon wafer exhibits strong reflection to both optical light (~40%) and THz radiation (~30%), which greatly limits the achievable tunability and versatility. Xie et al. verified that the modulation depth (MD) of silicon wafer to THz wave is only 19.9% under 800 mW femtosecond laser, although this value could be 98.6% if the pump laser intensity is high enough [21]. Very recently, plasmonic layers [22], graphene [23, 24], and even thin organic layers [25] have been fabricated on the surface of Si to enhance the modulation properties. Si modulators with these additional layers can work under lower pumping power, while having the same or even two to four times larger modulation depth. Very recently, Qi-Ye Wen and his co-workers demonstrated an interesting Si nanostructure for optically driven THz modulators [26]. They showed that nanotip (SiNT) arrays made from silicon wafer can be utilized as antireflection layers for both THz wave and visible light to achieve a low-loss and spectrally broadband THz modulator with a remarkably enhanced MD. Instead of fabricating heterogeneous materials on silicon, the nanotips are directly etched from the Si substrate and thus are structurally stable. Compared with the modulators fabricated on bare silicon, a nearly three times larger MD is achieved with the SiNT modulator. Crucially, the intrinsic THz transmission of the SiNT modulator is as high as 90% due to a strong antireflection effect arising from the nanotip layer as a result of the formation of graded refractive index on the Si surface.

One disadvantage of the semiconductor-based all-optical THz modulators is its relatively low modulation speed. The ultimate modulation speed is decided by the carrier recombination time of semiconductors, which for intrinsic or HR silicon is beyond 10 μs, limiting the modulation speed to a maximum of ~100 kHz. Though gallium arsenide (GaAs) has very short carrier lifetime (10–100 ps), it requires high-power pulsed laser excitation up to 1 kW/cm2 to achieve similar results as Si [27]. The reason for this is that the carrier lifetime of semiconductor affects the modulation depth and speed in an opposite way. Further research is ongoing to overcome this problem [27].

Another method of THz modulation is to thermally tune the electrical conductivity and thus the optical response of semiconductors or metal oxides, especial those materials with insulatormetallic phase transition [15, 28–31]. Vanadium dioxide (VO2 ), for example, is a typical phase transition material that exhibits a reversible first-order phase transition from an insulating state to a metallic state above room temperature (~68°C). Associated with this metal-insulator transition (MIT) is a lattice structural transition from the monoclinic to tetragonal, a change of conductivity by 3–5 orders of magnitude and significant changes of the optical properties at all wavelengths. Since the insulated state is transparent while conductive state is opaque to THz wave, the THz transmission can thus be dynamically modified from transparent to reflecting modes by controlling the phase transition of the VO2 film. VO2 films, separately or integrated with resonant element (e.g., metamaterials), have already been used to control and manipulate THz wave [15, 30, 32, 33].

Although great progress has been made in optically and thermally driven THz modulators, an all-electronic approach is more interesting and attractive for the real application. It is well known that the carrier concentration in semiconductors can be tuned by electric injection or depletion of charge carriers. It is proved that THz wave can be manipulated by the use of two-dimensional electron gases (2DEGs) in semiconductors [7, 8]. A semiconductor-based field-effect transistor (FET) is a very useful architecture to fabricate effective THz modulators.
