**4. Flexible THz modulator based on graphene FET**

In contrast to rigid THz modulators, flexible THz modulators are expected to be used in application fields with complicate surfaces [50]. A typical type of flexible modulator is a fieldgrating device, with which the intensity or phase of THz wave can be modulated by electrical gating or laser, but its properties remain unchanged under device deformation. This device is highly desired in nonplanar applications. Graphene is a highly flexibility material where its electronic structure can be maintained under deformation. Therefore, it is promising to develop flexible THz modulators based on graphene FET. Here, we give a typical example.

## **4.1. Device fabrication of the flexible THz modulator**

a central output in the 340 and a 240–400 GHz zero-bias Schottky diode intensity detector are included. In the measurement, we applied a square biasing voltage to the device and the output modulated THz waveform was recorded by an oscilloscope. The applied voltage pulse is −10 V at the minimum and 10 V at the maximum with various modulation frequencies. **Figure 4(a)**

**Figure 4(b)** shows the dependence of the normalized modulation magnitude on the modula-

constant of a transistor is an important parameter to determine the switch speed. The device resistance (R) was estimated to be 261 Ω by extracting the average resistance as the back voltage

film of 60 nm. The capacitance C is then calculated to be ~27.7 nF. As a result, the calculated *RC* time constant is ~138.7 kHz, which is very close to the directly measured 3-dB bandwidth (170

c

O3

film (~7.5), *ε*<sup>0</sup>




O3


) of 170 kHz. As we know, the RC time

/d, where ε is the


 and Al2 O3

O3

is the permittivity of free

, *d* is thickness of the Al2

O3

O3

(d) the comparison of the amplitudes of the THz wave transmission through the GFET modulators with SiO2

sweeping from −10 to 10 V. The capacitance (C) can be expressed by *C* = Aεε0

O3

space, *A* is the effective area of active graphene device of 5 × 5 mm2

the conventional silicon-based graphene THz modulators [39].

shows the recorded waveform of Al2

back gate voltage. The modulation depth of Al2

124 Design, Simulation and Construction of Field Effect Transistors

dielectric at 1 THz.

kHz). These results confirm that Al2

tion frequency, which gives rise to a 3-dB bandwidth (*f*

**Figure 3.** Normalized intensity of transmitted THz wave through the (a) SiO2

relative dielectric constant of the ALD-deposited Al2

The schematic diagram and photograph of the flexible THz modulator are presented in **Figure 5(a)** and **(b)**, respectively. The whole device is fabricated on a flexible commercial PET substrate. First, monolayer graphene was synthesized by typical chemical vapor deposition (CVD) on the copper foil and then was transferred onto the PET substrate [44–46]. The silver pastes were brushed at the two sides of graphene strip as the source and drain electrodes. The effective length and width of the channel of graphene FET was defined to be 2 and

**Figure 5.** (a) Schematic structure of the flexible THz modulator based on graphene coplanar-gate FET. (b) Photograph of the flexible THz modulator in the bending condition, where the boundary of graphene channel is marked with dotted lines.

1 cm, respectively. The ion-gel, which is a mixture of lithium perchlorate, polyethylene oxide (PEO), and carbinol, was spin coated on the surface of the graphene as the gate dielectric. The work principle of this coplanar-gate FET structure is: when a positive voltage to the device is applied, as shown in **Figure 5(a)**, negative and positive ions in the ion-gel accumulate onto the gate electrode and graphene channel, respectively. A strong electric field is thus imposed to the graphene to modulate the carrier concentration and as a result a modulation to the THz radiation is realized.
