Fibrous transistors

Textile electronic circuits based on organic fibrous transistors... 

The transistor as a component of the e-textile plays a crucial role in the textile electronic circuit. The existing fibrous transistors can be divided into two categories wire thin film transistors (WTFTs Lee and Subramanian, 2003 Maccioni et al., 2006 Locci et al., 2007) and wire electrochemical transistors (WECTs Hamedi et al., 2007 De Rossi, 2007 Tao et al., 2011). WTFT, also called WFET, is based on the field-effect transistor (FET) technology and WECT is based on electrochemical technology. With the help of these transistors, the textile electronic circuit can be achieved without loss of mechanical properties such as flexibility or softness. 

This chapter reviews the current status of the fibrous transistor development and textile electronic circuit. It covers semiconducting materials, fibrous transistors, and textile electronic circuits. Section 25.2 will discuss the materials employed in semiconductor manufacturing, especially in fibrous transistors. Section 25.3 will present traditional and fibrous transistors. Textile electronic circuits are discussed in Section 25.4. The different methodologies of use will be compared. And finally, the conclusion and perspectives will be discussed. 

Besides pentacene, some other small molecular organic semiconductors can also be rendered solution processable by attachment of flexible side chains. But they are no longer still used for realizing fibrous transistors. For more detailed information see (Sirringhaus, 2005). 

Transistors as the basic and crucial component for electronic circuits have been vigorously studied since 1947. The essential function of a transistor comes from its ability to control a larger signal by an apphed small signal between one pair of its terminals. This property makes the transistor as an amplifier or switch. According to the mechanism of the generation of carriers in the channel, transistors can be divided into three categories bipolar junction transistor, field-effect transistor, and electrochemical transistor. In electronic textile applications, the EET and ECT are employed to realize fibrous transistors. 

At the same time, anions are repelled to semiconductor side and another interface of electrolyte/semiconductor with high electric capacity is achieved. The double-layer capacitor induces the charge carrier in the semiconductor. The switch rate is lower than TFT because the accumulation of carriers in the semiconductor layer comes from the migration of ions in electrolyte, which is not instantaneous. Moreover, the electrolyte is in sol—gel form whose aging is still a problem in the case of no encapsulation. However, the easy processing condition is a huge advantage for fabrication of fibrous transistors. 

The same authors improved their fibrous transistor by using stainless wire as the gate electrode in order to ameliorate the interface quality between gate and dielectric... 

In 2004 another team of researchers developed a fibrous transistor by using Kapton fiber as the textile substrate shown in Fig. 25.13 (Bonderover and Wagner, 2004). The silicon nitride and amorphous silicon were deposited by conventional processing techniques, including conventional photohthography, at temperatures up to 150°C on the Kapton sheet. Then the sheet was sliced into the fibers. The threshold voltage was reported to be 7.5 V and the linear electron mobility was 0.13 cm s. However, the output characteristics were not shown in the report. We cannot know if there is leakage for this fibrous transistor. 

In 2006 Maccioni et al. developed another fibrous transistor by using stainless wire as gate and polyimide as dielectric. The novelty of this prototype is in the fact that they rotated the wire when they deposited the semiconductor (pentacene). As a result, the channel width was doubled and the semiconductor layer was more homogeneous, and the leakage of gate was tremendously improved. Besides, they used soft lithography by using PDMS stamp to realize the source and drain electrodes by PEDOT PSS. 

Figure 25.14 Schematic for fibrous transistor by using Mylar as insulator, pubbshed in (Bonfigbo et al., 2005).

The same research team published their theoretical modeling for the cylindrical OFET in 2007 (Locci et al., 2007). They considered the field-effect fibrous transistor as thin film transistor (Fig. 25.15) and reduced the source-drain current in linear and samration regimes as Eqs. [3.1] and [3.2] ... 

Under the hypotheses that no = N and rj ds, the threshold voltage is equal to the pinch-off voltage Vp = Vth, and all the terms in the second row of Eq. [3.2] can be neglected to obtain Eq. [3.3]. In this case, the field-effect fibrous transistor can be considered as a planar thin film FET. 

When Lee and Subramanian invented the first fibrous transistor (Lee and Subramanian, 2003), they did not use it to develop a textile circuit. The reason may be in the rigidity, stability, and reproductivity of the fibrous transistor. The wire was in aluminum. It was difficult to insert it into woven fabric as a normal textile filament. Moreover, its evaporated semiconductor and source-drain electrodes were delicate. They would be easily destroyed during the assembly process. Besides, the source-drain electrodes were only in one side of the wire. The alignment with other conductive yams was a problem in the source-drain position. Even if this kind of fibrous transistor was ameliorated by using stainless wire as gate and depositing the source-drain around the whole wire (Lee and Subramanian, 2005 Maccioni et al., 2006), there still was no laboratory prototype of a fibrous transistor-based electronic circuit published. Because the deposition should be carried out in vacuum, it became impossible to exploit series manufacturing for... 

On the other hand, the fibrous transistor-based circuit takes advantage of this miniature size and pure textile nature. Particularly for the WECT, they can be used to develop a basic digital or analogical circuit. The technical difficulties reside in the low work rate and flexibility. By using smaller ion-based electrolytes, the WECT will obtain a higher work rate. Meanwhile, the aging of sol—gel of electrolytes should be investigated. 

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