2.1.1. Transistors
Transistors are one of the key electrical components of integrated electronic circuits that amplify and convert electrical signals simultaneously. They can also be applied to a variety of optoelectronic devices, such as displays, integrated circuits (ICs), memory devices, synaptic devices, and sensors. To date, most transistors have been fabricated on planar and rigid substrates. The development of flexible fiber-shaped transistors to give fabrics computational ability is still at an early stage because of the ongoing challenges involved in forming uniform films and high-resolution patterning on fiber substrates, and interconnecting fiber electrodes
[2]. Thus, fabrication methods for electronic fibers and textile-compatible technology have been chosen as the primary focus of this research area.
Fiber-shaped transistors have mainly been fabricated using mechanically flexible materials with a coaxial structure. The dielectric, semiconductor, and electrode materials are sequentially coated onto an electrode core using solution (dip-coating, spray, etc.) and vapor processes (sputtering, atomic layer deposition, chemical vapor deposition, etc.)
[15][16]. Various semiconducting materials have been applied to create a fibriform channel layer, such as polymers
[17][18], organic small molecules
[3][15][19], single-walled carbon nanotubes (SWCNTs)
[17][20], and metal oxides
[21][22]. For instance, Kim et al. fabricated organic field-effect transistor (OFET) fibers on Au microwires (
Figure 1a)
[23]. The OFET fiber was made from a coaxial bi-layer composed of 2,8-difluoro-5,11-bis (triethylsilylethynyl)anthradithiophene (diF-TESADT) as an organic semiconductor and poly(methyl methacrylate) (PMMA) as the insulating polymer
[23]. The diF-TESADT: PMMA blend solution was coaxially die-coated and solidified with vertical phase-separation. The dimension of the spirally wrapped CNT microelectrodes was controlled using a rolling-transfer method, and the OFET fiber showed a maximum field-effect mobility of 0.68 cm
2V
−1s
−1 and good output current characteristics
[23]. 2D crystals of organic small molecules (2DCOS) were used to fabricate fibriform OFETs through a jigsaw-puzzle physical-chemical method
[19]. 2DCOS film was prepared using a solution epitaxy method, and then transferred onto a planar substrate to fabricate the OFETs
[19]. The 2DCOS-based FETs were then peeled off and attached to the target fibers. The 2DCOS fiber transistors showed competitive electronic characteristics: a high field-effect mobility of 1 cm
2V
−1s
−1, well-balanced ambipolarity via the p−n junction, high inverter gain up to 12.4, and a near-infrared photoresponsivity of 1.06 × 10
4 A W
−1, with photodetectivity of 10
13 Jones
[19]. Heo et al. reported reel-processed 1D complementary metal-oxide-semiconductor (CMOS) logic circuits based on SWCNT transistors (
Figure 1b)
[16]. P- and n-type SWCNT fiber transistors were demonstrated using selectively chemical doping and a photochemical patterning technique
[16]. The device exhibited high hole mobility of 4.03 cm
2V
−1s
−1 (electron mobility of 2.15 cm
2V
−1s
−1) and a gain of 6.76 with good dynamic operation at an applied voltage of 5.0 V
[16]. Park et al. also fabricated fiber-shaped FETs with an Al
2O
3−MgO nanolaminate insulator and an In−Ga−Zn−O (IGZO) semiconductor
[21]. The Al
2O
3−MgO and IGZO layers were deposited using a thermal atomic layer deposition system and radio-frequency sputtering, respectively
[21]. The resulting fiber-shaped IGZO FETs exhibited an on- and off-current ratio above 10
8 and good electron mobility of more than 3 cm
2V
−1s
−1 with a leakage off-current of less than 10
−9 A
[21].
Figure 1. (
a) Top: schematic of the rolling-transfer process of printed CNT microelectrodes (left) and photographs of the spirally wrapped CNT microelectrodes on fiber substrates (right). Bottom: schematic and photograph of a flexible fiber OFET (left), field-effect mobilities and on/off current ratio with various bending radii (right). Reproduced with permission Ref.
[23]. Copyright 2020, American Chemical Society. (
b) Left: schematic illustration of a fabrication process for fiber-shaped CMOS circuitry. Right: electrical properties of the 1D complementary inverters. Reproduced with permission Ref.
[16]. Copyright 2017, Wiley-VCH. (
c) Left: schematic illustration of a fabrication process of a fiber-shaped OFET with the twisted structure and a solid ion-gel electrolyte. Right: photograph and transfer characteristics of the fiber-shaped OFET embedded in a fabric. Reproduced with permission Ref.
[24]. Copyright 2019, Wiley-VCH. (
d) Left: schematic illustration and photograph of the ionogel-gated fiber transistor. Middle: transfer characteristics of the ionogel-gated transistor with different bending radii. Right: electrical characteristics of logic gate NAND (A and B curves: two logic gate inputs, Y curve: output from the circuit). Reproduced with permission Ref.
[20]. Copyright 2019, American Chemical Society.
Many obstacles continue to limit the practicality of using 1D FETs for electronic circuits. One of the main issues is that 1D FETs still require sophisticated fabrication techniques that demand a vacuum process such as thermal evaporation, sputtering, or atomic layer deposition, which is unsuitable for commercialization
[24]. Moreover, the high operating voltages required by OFETs, along with their low conductance values and unstable electrode interconnections, need to be resolved. In this regard, a new device design strategy has been developed for high-performance 1D FETs. Kim et al. fabricated fibrous OFETs with a twisted structure and a solid ion-gel electrolyte (
Figure 1c)
[24]. The source and drain (S/D) fiber electrodes were coated with an organic semiconductor and twisted together. The twisted assembly of electrode fibers was then surrounded by an ion gel, and the gate wire was then wound around that
[24]. The resulting fibrous OFETs achieved milliampere-level output current and a good on/off ratio of 10
5 at low gate voltages (below −1.3 V)
[24]. Their work reveals a promising structural strategy that could help overcome the current limitations of coaxial fiber FETs.
As another class of fiber transistors for wearable electronic devices, fiber-shaped organic electrochemical transistors (OECTs) have been explored because they can simplify the complex manufacturing processes required for coaxial fiber OFETs
[18][25]. The OECTs use an electrolyte instead of an insulating layer of FETs, and therefore they do not need a smooth fiber substrate
[25]. By applying a gate voltage, ions are injected from the electrolyte into the top surface or inner part of the semiconducting film, thereby doping the channel
[26]. However, OECTs operated by doping/de-doping conducting polymers can work only in the depletion mode, and the response time is longer than that of OFETs due to slow ion transport in the ionic liquid
[26]. Hamedi et al. demonstrated electric double-layer capacitor-gated (EDLC) transistors on sputter-coated metal fibers using poly(3-hexylthiophene) (P3HT) and imidazolium ionic liquid
[11]. In the EDLC-OFETs, the channel conductivity was modulated by electrolyte polarization upon exposure to an electric field
[11]. Therefore, the demonstrated EDLC transistors operated below 1 V and exhibited large current densities and improved switching speeds
[11]. Owyeung et al. also demonstrated CNT transistors on linen threads, using a colloidal silica-based ionic liquid gel that induced all-around electrostatic gating (
Figure 1d)
[20]. The thread substrate was knotted with S/D Au wire, and then the P3HT (or CNT) semiconductors and ionogel were sequentially deposited by drop casting onto the thread
[20]. These ionogel-gated transistors were applied as a switch and a multiplexed diagnostic device with simple logic gates (NAND, NOR, and NOT)
[20].
Electrochemical and electrolyte-gated transistors can easily be integrated into woven circuitry in textiles and operated at low voltages. However, the low reliability in switching behavior under chemical and bias stress must be improved before their application will be practical. For this reason, highly reliable electrolyte-based materials should be developed.