# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 6 Design and Simulation of LC-CNT-Based RF Devices  
Radio frequency switches are very important for an RF module in telecommunications and other technologies. Day by day. the demand for a high bit rate is increasing. Beside this, we need devices that can operate in a wide frequency range for multiband operations. 5G technology is on the way, so the demand of re-configurability is also increasing. For a good RF switch, we need a material which shows a large and sharp change in the resistivity for applied external stimuli (voltage, current, lights, etc.) like the phase change material, transitional metal oxide, etc. [101; 102; 103; 104; 105]. For a good RF device, a low ON resistance (\(\Omega\)s), a low OFF capacitance (fF), a low power consumption (\(\mu\)W-mW range), and a flat broad band response is required (up to 50 GHz). In this work, we designed and simulated an LC composite-based SPST (Single pole single through)-RF as a proof of concept. The configuration for our device is a series-type coplanar waveguide (CPW). In this kind of device, the switching material is inserted in the middle of the signal line. Usually, when the device is in the OFF state, the switching material is in an insulating state and no signal will pass through. When the switching material switches to a conducting state, the signal can go through. In the present case, the switching material is the 5 CB-CNT composite. Considering the fabrication process, we proposed two kinds of devices. In one fabrication process, at first, the gold line will be deposited using evaporation and the liftoff method (Figure 19). Then, these lines will be thickened to 10 \(\upmu\)m using electroplating. Finally, SU8 will be deposited in between the gold lines as a dielectric. In the second process, Deep reactive-ion etching (DRIE) can be used to make a trench on a SiO\({}_{2}\)/Si wafer (Figure 19). Then Au metal will be deposited to create the signal and ground lines. For simulation, we proposed a simple model of the device ignoring the parasitic. Here, the LC composite is modeled as a capacitor in parallel with a variable resistor as shown in Figure 20b. We also assumed that the capacitance does not vary much between the ON and OFF states. [PERSON] et al. demonstrated that, for a 3.3-\(\upmu\)m-thick cell with an area of approximately 50 mm\({}^{2}\), the OFF-state resistance is 1 M\(\Omega\) and the ON state resistance is 187 \(\Omega\)[57]. By optimizing the geometry, doping concentration, and host material, the ON-state resistance might be reduced to a few \(\Omega\)s. The typical value of the OFF state dielectric constant is around 5-10, with a cell spacing of 2-20 \(\upmu\)m and an area of 25-100 mm\({}^{2}\)[20; 57; 106]. Considering these values, we optimized the capacitance using the formula C = \(\frac{\varepsilon\varepsilon_{g}}{d}\). We found that, the OFF capacitance can be optimized to less than 0.05 pF (Figure 20a). Figure 19: A schematic of two device structures of a coplanar waveguide (CPW) RF device. Using the above values of resistance and capacitance, we simulated the RF device using the NI-multisim software. Figures 21 and 22 show the RF performance of the proposed devices. Figure 21 shows transmission characteristics with two different pairs of resistance and capacitance values. In both cases, the insertion loss shows flat characteristics. The insertion loss for R\({}_{\text{on}}\) = 5 \(\Omega\) and C\({}_{\text{off}}\) = 0.05 pF is 0.95 dB over a frequency range of up to 10 GHz, whereas the isolation is greater than 30 dB up to 10 GHz. On the other hand, the return loss for R\({}_{\text{on}}\) = 50 \(\Omega\) and C\({}_{\text{off}}\) = 0.5 pF shows a decreasing response with frequency. As the R\({}_{\text{on}}\) and C\({}_{\text{off}}\) values decreases, the response became flatter as shown in Figure 22b. For R\({}_{\text{on}}\) = 5 \(\Omega\) and C\({}_{\text{off}}\) = 0.05 pF, the return loss is 60 dB up to 10 GHz. This means that signal reflection is much less in the ON state.