Development and Evaluation of Thread Transistor Based on Carbon-Nanotube Composite Thread with Ionic Gel and Its Application to Logic Gates

Abstract

We propose a new type of flexible transistor based on carbon-nanotube (CNT) composite thread (CNTCT), i.e., a thread transistor, with ionic gel. In our previous study, we demonstrated that transistor operation was possible by combining metallic and semiconducting CNTCTs as gate and channel with an insulating material. However, its performance was not sufficient. Therefore, we here aim to improve it. For this, we tried to apply ionic gel as a dielectric layer to it. With this, the transistor was expected to be an electric-double-layer transistor. The transistor performance was improved, and the on/off ratio of the transistor increased by more than 4. This is a large value compared to our previous work. In addition, we not only evaluated the performance of the transistors, but also investigated whether they could be used as logic circuits. It was confirmed that the logic circuit composed of the thread transistor also operated correctly and stably for a long period of time. It was also confirmed that the output changed in response to weak external forces. These results indicate that it is a flexible transistor that can be used in a wide range of applications such as logic circuits and sensors.

1. Introduction

Nanotechnology research has made significant progress in recent years. Within these research areas, nanocarbon materials such as fullerenes, carbon nanotubes (CNTs), and graphenes have been studied in a wide range of fields, from fabrication techniques to applications. Especially CNTs, discovered in 1991 [1], are known for their high chemical stability, mechanical strength, high electrical and thermal conductivity, and metallic and semiconducting properties [2,3,4,5,6,7]. The unique electrical properties of CNTs are due to their structure. CNTs have cylindrical structures derived from graphene, and their energy band changes depending on their structures. That is, the structure affects their electrical property. It is known that single-walled CNTs show semiconducting properties when the chirality parameter (n, m) satisfies the condition 2n − m = multiple of 3, and multiwalled CNTs generally show metallic properties [6]. By selecting the CNTs to be used based on the above, it is possible to develop composite materials and applications with the desired electrical properties. Moreover, advancements in separation technology for CNTs have made it relatively easy to obtain a high purity of semiconducting CNTs and high purity of metallic CNTs, respectively. Due to the various beneficial properties mentioned above, the practical application using CNTs in various applied products is highly awaited [8]. As one of the application research, CNT transistors utilizing the semiconducting properties of CNTs have long been studied [9,10,11], and a wide range of applications are being pioneered, including sensor applications [12,13,14,15] and the development of thin-film transistors [14,15,16,17]. However, CNTs are generally very small, with a diameter of only a few nm and a length of only a few mm, and most commercial products are in powder form or aqueous dispersion, making them difficult to handle as they are and making it difficult to develop applications. To solve this problem, studies are being conducted not only on how to use CNTs alone [18,19,20], but also on how to apply them to devices in the form of composite materials combined with other materials [21,22,23]; mixing CNTs with other materials and treating them as “CNT composite materials” is one solution. By forming composite materials, CNTs can be easily handled, and their functions can be applied in that form [24,25,26,27].

We have been developing “CNT composite threads [28]” that are easy to handle as “familiar objects” taking advantage of the various features of CNTs. This composite material has attracted attention as a unique new material because it has the same processability and deformability as threads with maintaining the various functions of CNTs. Many applications of this composite material, such as “thermoelectric power generating threads [29]” and “Peltier threads [30]”, have already been studied and found to be feasible. In addition, we have been developing a unique transistor named “thread transistor [28]”, which is a combination of a semiconducting CNT composite thread, a metallic CNT composite thread, and an insulating material. In our previous study [30,31], the feasibility of the transistor has become clear, but its performance was still insufficient. In this study, therefore, we tried to improve the performance of the transistor by applying ionic gel to the gate part of the thread transistor and constructing an electrical double layer. In addition to this, we proceeded with the evaluation through the circuit configuration and measurement of logic gates, which did not exhibit the desired operation in previous studies.

We believe that by realizing the proposed thread transistor, it will be a new type of a flexible transistor. Since this takes the form of thread, a familiar material, it is expected that it will be used in our daily life in the near future.

2. Materials and Methods

2.1. Structure and Operating Principle of Thread Transistor

Figure 1 shows the structure of our thread transistor based on our CNT composite thread. The thread transistor is a three-terminal device consisting of gate, source and drain terminals, and the drain current can be controlled by applying gate voltage, just like a general metal-oxide-semiconductor field effect transistor (MOSFET). As explained in the Introduction, CNTs have metallic and semiconductive properties. Thus, we can prepare metallic and semiconducting CNT composite threads, respectively. A metallic CNT composite thread is used as the gate electrode, a semiconducting CNT composite thread is used as the channel, and an ionic gel is used as a gate dielectric layer. The ionic gel is an electrolyte gel in which an ionic liquid and a polymer form a three-dimensional network structure. Unlike general MOSFETs, the transistor operates as an electric double layer (EDL) transistor [32] by using an ion conductive electrolyte such as ionic gel as gate dielectric layer. In an EDL transistor, the gate voltage acts indirectly on the channel. In other words, the gate voltage causes the ions inside the gate dielectric layer to move and align at the semiconductor interface. The ions that align in a very short distance to the semiconductor interface form the channel, and the transistor operates. Since the distance between the aligned ions and the channel in an EDL transistor is much shorter than the distance between the gate electrode and the channel in a MOSFET, the EDL transistor can be driven at a lower voltage than a typical MOSFET. In summary, the advantage of choosing the EDL transistor type is to make the gate more effective. Our previous study used a non-EDL transistor type structure, but the controllability by the gate was very weak [28,31]. By choosing the EDL transistor type in this study, we aimed to solve this issue.

Figure 2 shows extracted three layers from the gate electrode to the channel parts of the transistor, and shows the behavior of the charge when an electric field is applied. Figure 2a shows a schematic diagram before the gate voltage application. As shown in the figure, the anion and cation (EMI⁺ and TFSI⁻ in this study as described in the following Section 2.2.2) in the ionic gel are in random position in the electrolyte. By applying gate voltage as shown in Figure 2b, ions in the electrolyte begin to move and attract the charge on the electrode. After applying the gate voltage as shown in Figure 2c, each ion moves to the gate electrode interface and cannel interface, respectively, and aligns with the electrode at a very short distance of about 1 nm (this distance can be estimated by the ion size used [33,34]). This is the EDL. These two parallel electric double layers can be considered as a series connection of capacitors, so that transistor acquires a high specific capacitance (>1 μF/cm). As a result, it is known that carriers can be introduced at the channel interface at low voltage [35].

2.2. Preparation and Fabrication of Thread Transistor

The thread transistor can be made by combining the metallic CNT composite thread, the semiconducting CNT composite thread, and the ionic gel (Figure 3). These materials function as the gate electrode, the channel, and the gate dielectric layer in the proposed thread transistor, respectively. It can be fabricated in a simple method without the need for large-scale apparatus.

2.2.1. CNT Composite Thread-Making Method

The CNT composite thread can be made by a simple method such as the traditional dyeing method [28,29,30,31]. This is a simple method that involves dipping a thread in the CNT dispersion and drying in oven.

The making process of the CNT composite thread is described as follows:

• Amounts of 10 mg of CNT and 100 mg of sodium dodecyl sulfate (SDS) were dispersed in 20 mL of pure water with an ultrasonic homogenizer (UX-50, Mitsui Electric Co., Ltd., Tokyo, Japan) for 1 h. We chose multi-walled CNTs (NC7000, Nanocyl, diameter = 9.5 nm) to prepare metallic CNT dispersion and chose single-walled CNTs (SG65i, CHASM, (6,5)-chirality, 95%purity, diameter = 0.78 nm) to prepare semiconducting CNT dispersion in this study.
• Then, 10 cm of cotton thread (diameter = 0.4 mm) was prepared. This was washed lightly with pure water, placed on a glass plate, and $100 \mathsf{\mu }\mathrm{L}$ of the dispersion was dropped so that the entire thread was covered with the CNT dispersion.
• The glass plate was put in an oven at 60 °C to fix the CNT on/in the thread. This was left it in an oven for about 30 min until the dispersion was completely evaporated.
• The thread was removed from the glass plate and washed with pure water to residual surfactant.

CNT composite thread can be fabricated by the above process from steps (1) to (4). By dyeing the thread with the CNT dispersion, the CNTs are fixed to each fiber of the thread as shown in the SEM image of Figure 4. Cellulose (the main component of thread) and CNTs have a rectilinear structure, and CNTs are fixed by a large van der Waals force. To increase the amount of fixed CNTs on the thread, steps (2)–(4) must be repeated multiple times. The amount of CNTs and impurities should be considered depending on the application. In thread transistors, we dyed 5 times for metallic CNT composite threads and dyed 2 to 3 times for semiconducting CNT composite threads. Regarding impurities of semiconducting CNT composite threads, we used commercially available (6,5)-chirality CNTs of 95% purity in their untreated state and we remove surfactant used to disperse CNTs by washing the composite thread with pure water. It would be difficult to remove all of the surfactant by washing, so the purity of the semiconducting CNT is probably lower than 95%, but we believe it is close to that.

2.2.2. Ionic Gel-Making Method

An ionic gel is an electrolyte gel in which ionic liquid and polymers form a three-dimensional network structure. Ionic liquid is formed of salts composed of anions and cations, and is usually in liquid form at room temperature due to its extremely low vapor pressure.

Although there are some studies that use the ionic liquid itself as gate dielectric layer [36], in this study, the ionic liquid is gelatinized and used as the gate dielectric layer. Gelation has the advantage of physically connecting the two threads, improving their flexibility.

In this study, the materials were selected to produce ionic gels that are easy to fabricate and show good performance. For the ionic liquid, we chose EMI-TFSI (1-Ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl)imide), which has a high ionic conductivity of $8.4 \mathrm{mS}/\mathrm{cm}$ and is widely used as a gate dielectric layer in transistors [37,38]. For the polymer, we chose PVDF-HFP (poly (vinylidene fluoride-hexafluopropylene)), which has a high dielectric constant ($\epsilon =8.4$) and can hold a large amount of ionic liquids due to the amorphous property of HFP. Typical polymers that can be used to prepare ionic gel in simple process are PVDF, PVA (poly (vinyl alcohol), PMMA (poly (methyl methacrylate), and so on [39]. This is the first trial to fabricate ionic gel in this study, and PVDF (typical polymer) was selected as the first step in this approach. And we selected PVDF-HFP because of its non-crystalline HFP region, which offers the advantages of liquid holding performance, flexibility, and low melting point as reported in [40]. It is known that PVDF-HFP has many unique applications such as ionic thermoelectric devices [41]. When this research progresses, we believe that the use of this ionic gel could extend the functionality beyond transistors.

And for the solvent, we choose acetone because it is widely used and has a low boiling point of $56.5 °\mathrm{C}$.

The making process of ionic gel is described as follows.

• An amount of 20 mg of PVDF-HFP (Sigma-Aldrich, St. Louis, MO, USA, average Mw$~\mathrm{400,000}$, Mn$~\mathrm{130,000}$) was dissolved in 5 mL of acetone by heating and stirring at $70 °\mathrm{C}$ for 3 h in a magnetic stirrer (SP88854200, Fisherbrand, Waltham, MA, USA).
• An amount of 1 mL of ionic liquid (EMI-TFSI, TOYO GOSEI Co., Ltd., Tokyo, Japan) was added to the polymer solution prepared in step (1); then, heating and stirring were conducted at $70 °\mathrm{C}$ for 3 h, and acetone was evaporated.

The completed ionic gel is a thermo-reversible gel. As shown in Figure 5, the ionic gel softens when the gel is heated to about $100 °\mathrm{C}$. And the gel becomes hard when it is cooled.

2.2.3. Thread Transistor-Making Method

As explained in Section 2.1 and the beginning of Section 2.2, the proposed thread transistor can be made with a very simple process. The concrete-making process of the proposed thread transistor is described as below:

(1)

In preparation, the ionic gel was heated to $120 °\mathrm{C}$ to gelatinize it. Then, 1 cm of metallic CNT composite thread and semiconducting CNT composite thread were cut.

(2)

The gel was attached to the tip of the metallic CNT composite thread.

(3)

Before the gel cooled and hardened, it was physically connected to the center of the semiconducting CNT composite thread prepared in step (2).

(4)

The fabricated thread transistor was dried in an oven at $80 °\mathrm{C}$ for 30 min.

The fabricated thread transistor is shown in Figure 6. The size of the thread transistor was as follows: gate electrode thread (metallic CNT composite thread) $L_G=1 \mathrm{cm}$, channel thread (semiconducting CNT composite thread) $L_{SD}=1 \mathrm{cm}$, length of thread covered by gel $L=4 \mathrm{mm}$, width of thread covered by gel $W=4 \mathrm{mm}$. When ionic gel is used as the gate dielectric layer, the transistor operates by the formation of the EDL as explained in Section 2.1. That is, when the gate voltage is applied, the ions contained in the gel move to the gate electrode surface and semiconductor surface to form EDL. Carriers in the semiconductor are attracted by this EDL to form a channel. Therefore, the semiconductor area in contact with the ionic gel is defined as the channel length and channel width, respectively [42].

The thread transistor has a channel covered all around, forming a gate all-around structure. This structure allows gate voltage to be applied to the entire channel, efficiently modulating drain current. The flexibility of the thread transistor depends on the durability of the gel. If the thread is pulled with a strong force, the gel tears off at 0.35 N and the transistor is broken, but it shows high flexibility under weak external force.

2.3. Performance Evaluation Method of Thread Transistor

In this study, we conducted the following four experiments.

• Investigation of the effect of applying tension to a CNT composite thread:

To evaluate the mechanical properties of the produced CNT composite threads, the following measurements were performed. First, 2 cm of the CNT composite thread was prepared and connected to a digital multimeter (TY720, Yokogawa Test & Measurement Corporation, Tokyo, Japan). Then, a force gauge (AD-4932A, A&D Co., Ltd., Tokyo, Japan) was connected to one end of the thread. The thread was pulled from the other end, and measured the tension applied to the thread and resistance at the same time.

• Measurement of thread transistor characteristics:

To evaluate the electrical characteristics of the configured thread transistors, the following measurements were performed. Three terminals of thread transistor as shown in Figure 6 were connected to leads, respectively, and the transistor characteristics were measured by using a semiconducting parameter analyzer (Semiconductor Characterization System, KEITHLEY, 4200A-SCS, Solon, OH, USA).

• Measurement of logic gate operation:

As one application of thread transistors, we tested the feasibility of constructing logic gate elements. Specifically, a NOT gate, the most basic and simple logic gate, was constructed using our thread transistor. As reported in our previous study [31], our thread transistor operated as a p-type transistor under atmospheric conditions, and since there are still issues in realizing an n-type transistor, for simplicity, we used a NOT gate circuit with a p-type thread transistor and resistors in this study. The evaluation method of the gate circuit is shown below. After constructing a NOT circuit consisting of a single thread transistor and a register, an input voltage $V_{IN}$ was applied using an arbitrary function generator (FGX-2220, TEXIO TECHNOLOGY CORPORATION., Kanagawa, Japan), and a bias $V_{DD}$ was also applied using a DC power supply (E3612A, Keysight Technologies, Santa Rosa, CA, USA). The output voltage was measured using a digital oscilloscope (DS-5105B, IWATSU ELECTRIC Co., Ltd. Tokyo, Japan). Sustainability was measured using a digital multimeter.

• Investigation of the effect of applying tension to a thread transistor:

Finally, the response of the transistor to bending and tension was evaluated. $V_{DS}=V_{GS}=-2V$ was applied to the transistor with a DC power supply, and the drain current was measured with a digital multimeter when an external force was applied. For bending, the transistor was fixed with carbon tape (NISSHIN EM Co., Ltd., Tokyo, Japan) and deformed with a bending radius of about 1 cm. In tension, the transistor was fixed vertically and deformed with weight.

3. Results and Discussion

3.1. Flexibility of CNT Composite Thread

Our CNT composite thread has the metallic or semiconducting properties of CNTs while being a flexible material, because the CNTs are fixed to each fiber of the thread. Here, we measured the ratio of resistance change when tension was applied to the CNT composite thread by the method described in Section 2.3 (A). The results are shown in Figure 7. Figure 7a shows the ratio of resistance change when tension was continuously applied from 0 N to 2.0 N. The resistance changed sensitivity to even small external forces, and different resistance levels were obtained for each external force. The resistance value changed significantly up to 1.0 N, and then changed slowly thereafter. It is thought that when weak tension was applied, the internal structure of the CNT composite thread deformed, and the contact of the CNTs fixed to the fiber formed a conductive path, resulting in a decrease in resistance. When stronger tension was applied, the CNT composite thread itself was deformed, resulting in a smaller diameter and possibly reduced resistance. Figure 7b shows the ratio of change of resistance when tension was applied from 0 N to 0.2 N and then released from tension. It can be seen that the thread showed different responses to small tensions, and when the tension was released, it returned to its original resistance value before the tension was applied. This result indicates that the CNT composite thread is a flexible material with low hysteresis property. Figure 7c shows the ratio of resistance change when the operation in Figure 7b is repeated five times. The resistance change was observed to be stable even after five stretching operations; thus, it can be confirmed that the thread has excellent cycle stability. This indicates that the CNTs were firmly fixed to the thread fiber, rather than filming and fixed to the thread surface.

In general, the ratio of resistance change of metallic CNT composite thread using multi-walled CNTs is larger than that of semiconducting CNT composite thread. This is because CNTs with larger diameter have a greater probability of CNT contact due to tension, resulting in a larger resistance change. In summary, CNT composite threads show excellent flexibility and responsiveness, and are considered to be suitable materials for mechanical sensing in wearable devices, for example.

3.2. Characterization of Thread Transistor

We measured the I-V and the transfer characteristics of the thread transistor by the methods described in Section 2.3 (B) (Figure 8). The fabricated thread transistor showed an increase in drain current depending on the negative gate voltage. It is known that semiconducting CNTs become p-type because of oxygen in the atmosphere, and the thread transistor with semiconducting CNTs at the channel also operates as a p-type transistor.

We evaluated the on/off ratio. The on/off ratio is calculated as the ratio of the output current when the transistor is in the on-state to the output current when it is in the off-state, as can be generally known. The respective currents can be easily obtained by reading the transfer curve, but our thread transistor in this study has high current even at gate non-bias, and it is difficult to confirm the saturation region due to impurities. Meanwhile, when the drain voltage was −2 V, the increase rate of the current slightly decreased and appears to shift towards the saturation region. If the contained impurities are reduced in future research, there is a possibility that the saturation region can be confirmed more definitely. So, the calculation was performed in a simplified method at this time. In other words, the ratio of the maximum current to the minimum current was calculated at a drain voltage of −2.0 V, at which the ion gel operates electrochemically stably.

The transistor was able to obtain an on/off ratio of 4.1, which is the modulation ratio of the drain current at a gate voltage of −2.0 V and a drain voltage of −2.0 V. This is a large value compared to our previous studies. This result may be due to the fact that the ionic gel used as the gate dielectric layer formed an EDL and was able to easily modulate the drain current. From this, we succeeded in developing a CNT composite thread that was successfully used in the operation of an EDL transistor. The point at which the gate voltage deviates from 0 V has also been reported in studies of CNT-FETs with EDL [43]. Therefore, in actual use, this voltage shift should be taken into account, or ion gels with less shift should be found and applied.

In contrast, the remaining issues are that the I-V curve varies linearly, and the off-state current is as high as $-33 \mathsf{\mu }\mathrm{A}$. It seems that the thread transistor does not show a clear linear region and saturation region. It is thought that the drain current increases even in the saturation region. This is due to impurities in the CNT composite thread. The impurities in the semiconducting CNT composite thread are metallic CNTs, which are estimated to contain about 5% in commercial (6,5)-chirality CNTs, and SDS, which was used to disperse the CNTs. In particular, the presence of metallic-type CNTs is thought to affect the off-current. The removal of these impurities is expected to reduce the off-state current.

3.3. Logic Gate

A p-type NOT circuit using the thread transistor was constructed by the method described in Section 2.3 (C), and the output, repeatability, and frequency response of the NOT circuit were measured. Figure 9 shows the results. Figure 9b shows the output of the constructed p-type NOT circuit. The output voltage changed slowly in response to the input voltage, which is the desired output of inverting the input signal. Since the operation of the EDL transistors is by ion transfer, the output (response speed) of the logic circuit is known to be slow [43], and thus the obtained response in this study is correct. Figure 9c shows the repeatability of the operation as the NOT circuit. The constructed NOT circuit was confirmed to operate stably for more than 10,000 s. The amplitude gradually decreases with operation, but becomes stable after 3000 s. The decrease in amplitude is considered to be due to remaining acetone, and the output voltage stabilized after the acetone evaporated and the gel structure stabilized. Figure 9d shows the change in output voltage at different frequencies. It is known that the operation of the EDL transistor using ionic liquids is limited in operating frequency due to its requirement to form EDL. If the ionic liquids themselves are used as dielectric layers, they are stable up to about ${10}^6$ Hz. However, when ionic liquids are combined with polymer to form a gel, the operating frequency is limited to 10 Hz because ionic migration is hindered [40]. In this study, the operating frequency of the thread transistor is around 1 Hz, and we believe that 10 Hz operation will be possible in near future. Since the output amplitude can be increased by improving the on/off ratio of the thread transistor, it is expected to operate as a NOT circuit at higher frequencies. In addition, we believe that other approaches, such as evaluating the operation of thread transistors when using other applicable polymers, as described in the Introduction, would also be effective.

We have successfully operated a simple logic circuit using the thread transistor fabricated in this study. It is expected that more complex logic circuits, such as those combining multiple transistors, will be applied in future research.

3.4. Responsiveness to External Forces

As shown in Section 3.1, the CNT composite thread is sensitive to tension, and its resistance changes when tension is applied. Since the thread transistor is composed of the CNT composite threads, it is thought that the resistance changes with tension, similar to the results shown in Section 3.1, and the drain current changes as a result. In this section, we investigated how the output current of the thread transistor changes in response to bending and tension by the method described in Section 2.3 (D).

3.4.1. For Bending

To investigate the change in output of the thread transistor in response to bending, it was fixed to the base paper using carbon tape, as shown in Figure 10a. The thread transistor was deformed together with the base paper, and the change in output during the deformation was observed. Figure 10b shows the change in drain current when bending in the channel direction. By bending the CNT composite thread, the resistance decreases, resulting in an increase in the drain current. Bending the thread transistor under the experimental conditions increases the output by about $10\%$ even though the gate voltage was fixed, and the output returned to the original level when the bending was returned. The response to bending was reproducible all three experiments conducted, and also showed excellent flexibility. Figure 10c shows the change in drain current when bending in the gate direction. The drain current changed in response to the deformation, but clear repeatability could not be confirmed. This is because there is no external force applied to the channel part as in the experiment shown in Figure 10b, resulting in almost no resistance change. In addition, the change in the distance between the gate and the channel, which is reduced by the bending, is limited, and as a result, the change was not observed as in Figure 10b.

3.4.2. For Tension

To investigate the change in output of the thread transistor in response to tension, it was fixed to the prepared pins on the breadboard, as shown in Figure 11a,b. Figure 11c shows the measured change in drain current when tension was applied to the thread transistor in the channel direction as in Figure 11a. By applying tension in the channel direction, an output corresponding to magnitude of the tension was obtained. When the tension is released, the drain current returns to the original drain current with small hysteresis. The increase in the drain current for tension as small as 0.04 N suggests that the thread transistor could be used as a high-precision tension sensor. Figure 11d shows the measured change in drain current when tension was applied in the gate direction as in Figure 11b. The results show some response to tension, but not much dependence on the magnitude. For tensions up to 0.08 N, the response also appears to increase proportionally, but for higher tensions, the trend of change changes. This is thought to be because the increased tension destroys the ionic gel at the tip of the gate electrode, resulting in a significant degradation of the performance of the transistor. In fact, it was confirmed that when the gate was tensioned with stronger force, the threads eventually separated from each other, and eventually, the transistor structure was destroyed.

4. Conclusions

In this study, a new thread transistor using a CNT composite thread with ionic gel was developed. In our previous studies, metallic CNT composite threads, semiconducting CNT composite threads, and insulating materials were combined to make thread transistors; however, their performance was not sufficient. In this study, to improve performance, a new thread transistor was developed using ionic gel as the gate dielectric layer. By using ionic gel, the thread transistor becomes the EDL transistor.

The feature of the EDL transistor is that EDL is formed in the gate dielectric layer part, which allows drain current modulation even at low gate voltage. In this paper, we reported that we were able to fabricate a transistor with an on/off ratio of more than 4. This is a large value compared to our previous work. In addition, we not only evaluated the performance of the transistors, but also investigated whether they could be used as logic circuits. It was confirmed that the logic circuit composed of the thread transistor also operated correctly and stably for a long period of time. It was also confirmed that the output changed in response to weak external forces. These results indicate that the transistor is a flexible transistor that can be used in a wide range of applications such as logic circuits and sensors. However, this research has revealed two issues. The first is that impurities in the semiconducting CNT composite threads increase the off-current output, resulting in a low on/off ratio. The second is that the ionic gel part is brittle against strong external forces and needs to be examined to overcome strong external forces. If these problems are solved and studies are further advanced, it is expected that the highly flexible thread transistor developed in this study will be used in our daily lives in the near future.