1.
Introduction
Flexible electronics has received increasing attention in recent years because it offers a number of novel applications concerning multiple areas such as flexible circuits[1, 2], implantable medical devices[3], flexible displays[4–9], electronic textiles[10], electronic paper[11, 12], wearable devices[13–15], conformable radio frequency identification devices (RFID)[16–18], and electronic skin for robots[19–21]. Flexible electronics enables electronic systems to be compact, light-weight, ultra-thin, stretchable, conformable, or even bio-compatible[22]. Thin film transistors, one of the most common active thin film devices which form the foundation of flexible electronics, are significant elements in flexible platforms[16]. Conventional silicon-based thin film transistors (TFTs) are widely used in flat panel displays. Flexible amorphous silicon (a-Si) thin film transistor devices have been investigated as driving elements in bendable displays[23, 24]. In addition, considerable advances have been made in flexible organic thin film transistors (OTFTs) on account of their inherent flexibility[25–27]. However, organic semiconductors exhibit relatively low mobility and stability[28, 29].
Since the demonstration of amorphous indium-gallium-zinc-oxide (a-IGZO) based transparent flexible TFTs in 2004[30], flexible oxide-based TFTs have attracted a great deal of attention in the field of flexible electronics[31, 32]. Compared with conventional amorphous silicon, oxide-based TFTs exhibit high carrier mobility, low off-current, low process temperature, and high transparency in the visible region[30, 33–36]. The low process temperature enables oxide-based TFTs compatible with flexible substrates such as paper and plastics. Moreover, amorphous oxide materials are compatible with traditional semiconductor processing techniques such as sputtering and patterning by photolithography, and their amorphous state ensures the device uniformity over large areas[37–39]. Flexible sensors[40, 41], memories[42, 43], circuits[44–46], and displays[47–50] implemented by oxide-based TFTs on compliant substrates have been widely investigated. Fig. 1 shows the possible applications for flexible oxide-based TFTs.
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Figure1.
(Color online) Possible applications for flexible oxide-based TFTs. The inset images were reproduced from Refs. [15, 25, 30, 45, 50–74].
2.
Oxide semiconductor materials
Thin film transistors were firstly proposed in 1962[75] and have been widely used in flat panel displays. Oxide-based TFTs have attracted a great deal of attention due to their low process temperature, high carrier mobility, and good uniformity. In the past few years, oxide semiconductor materials have been widely investigated. Here, a brief introduction of oxide semiconductor materials is provided.
N-type oxide semiconductors such as zinc oxide (ZnO), tin oxide (SnO2), and indium oxide (In2O3) as well as their combinations, indium–zinc–oxide (IZO), indium–tin–oxide (ITO), zinc–tin–oxide (ZTO), and indium–tin–zinc–oxide (ITZO) have been widely investigated recently[34, 35, 37, 76–83]. Oxide semiconductors exhibit high transparency in the visible region because of their wide band-gap (>3.0 eV)[84]. Transparent oxide semiconductors, ITO for example, are usually used as transparent electrodes in flat panel displays[85].
Ga-doped IZO (IGZO) in a crystalline phase and amorphous state were proposed by Nomura et al. in 2003[86] and 2004[30], respectively. A field effect mobility of ~10 cm2V?1s?1 was obtained in a-IGZO TFTs. As an active channel layer of TFTs, amorphous semiconductors are superior to polycrystalline ones due to their lower process temperature and better device uniformity. However, amorphous silicon which is widely used in flat panel displays typically exhibits field effect mobility lower than 10 cm2V?1s?1[87]. High mobility (>10 cm2V?1s?1) is possible for amorphous oxide semiconductors which contain post-transition-metal cations[88, 89]. Fig. 2 shows the carrier transport paths in amorphous oxide semiconductors. Polycrystalline phase hafnium-indium–zinc–oxide (HIZO), amorphous zirconium-indium–zinc–oxide (ZrInZnO), high mobility oxide/carbon nanotube composite, and polymer-doping amorphous oxide material were also investigated[73, 90–92].
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Figure2.
(Color online) Schematic orbital drawings of (a) covalent semiconductor, and (b) metal oxide semiconductor in crystalline and amorphous states. Reproduced from Ref. [30].
Complementary inverters based on both n- and p-channel transistors are basic building blocks for low-power and high-performance integration. Most of the oxide semiconductors exhibit n-type channels, which are suitable for switching/ driving TFTs in next generation flat panel displays and bendable displays. P-channel oxide-based TFTs are still requisite for achieving various low-power circuits. Zinc oxide is an attractive material in many application areas such as optical devices, acoustics, and sensing. However, reproducible and stable p-type doping for ZnO is a bottleneck for its commercialization[93]. The developments of high performance p-type oxide semiconductors such as ternary Cu-bearing oxides, binary copper oxides, tin monoxide, spinel oxides, and nickel oxides greatly enlarge the family of oxide semiconducto rs[46, 70, 94–100].
A variety of methods such as pulse laser deposition (PLD)[30], magnetron sputtering[2, 8, 17, 18, 36], chemical vapor deposition (CVD)[101], atomic layer deposition (ALD)[102], electron beam evaporation[2, 103], and solution processes including spin coating and printing[38, 104–110] are usually applied to deposit oxide semiconductor materials. Printing electronics offers a low-cost process technology that patterning and depositing can be simultaneously accomplished. Sputtering and PLD are the most frequently used oxide semiconductor deposition methods at room temperature. Atomic layer deposition, which is based on self-limiting and saturation surface reaction, offers one greatly controllable chemical deposition method at a relatively low temperature of ~150 °C.
3.
Flexible oxide-based TFTs on various substrates
In order to achieve flexible devices, bendable substrate materials must be prepared. Oxide-based TFTs with low process temperature are well compatible with flexible substrates. A lot of works were reported on flexible oxide-based TFTs, and here we introduce some of them classified by the flexible substrates including polymer plastics, paper sheets, metal foils, and flexible thin glass.
3.1
Polymer plastics
Polymer plastics with plenty of advantages such as light-weight, high transparency, conformable, stretchable, and bendable are appropriate for flexible substrates[30, 111–113]. Amorphous oxide-based TFT technologies with low process temperature are compatible with polymer plastics[114].
Among a large variety of polymer plastics, polyimide (PI) is the most frequently used plastic substrate for flexible oxide-based TFTs[17, 115–138]. In addition, other polymer plastic foils such as polyethylene naphthalate (PEN)[52, 71, 139–151], polyethylene-terephthalate (PET)[152–164], polycarbonate (PC)[165–167], polyether-sulfone (PES)[168–172], polydimethylsiloxane (PDMS)[42, 173, 174], polyarylate[175], and poly(vinyl alcohol)(PVA)[176] are also usually used. Glass-fabric reinforced composite films are also fabricated as flexible plastic substrates[177]. Polyimide precursor and polyimide-based nanocomposite coated on the glass carrier are also used as flexible substrates[178–182]. After the fabrication process, the polymer plastic films were released from the glass carriers to yield the free-standing flexible TFTs. To ease the detachment process, a release layer was deposited on the rigid carrier before the polyimide was coated[183–185].
Plastic materials with thickness from a few micrometers to several hundred microns are commercially availab le[186–189]. Flexible oxide-based TFTs can be directly fabricated on the free-standing polymer plastic substrates[190–192]. In some situations, plastic substrates are attached to rigid carriers by cool-off type adhesive or other glues for mechanical support during the fabrication process and detached from the carriers when the device formation is completed[193–196]. In some cases, oxide-based TFTs are fabricated on rigid substrates and then transferred to compliant substrates[197, 198].
In 2004, Nomura et al. demonstrated transparent flexible a-IGZO TFTs with PET substrate for the first time[30]. Fig. 3 shows the construction of the device and a photograph of flexible TFTs under bending condition on the probe station. The channel layer (IGZO), gate dielectric (Y2O3), and electrodes (ITO) (gate, drain, and source) were all deposited by pulse laser deposition at room temperature. Thin film transistors processed in this way exhibited good performance such as saturation mobility of ~6–9 cm2V?1s?1, a low leak current of about 10?10 A, and on/off ratio of about 103 even during and after bending.
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Figure3.
(Color online) (a) The structure of transparent flexible IGZO TFTs. (b) A photograph of the flexible TFT bent at R = 30 mm on the probe station. Reproduced from Ref. [30].
Park et al. studied flexible IGZO TFTs on 125-μm-thick PET substrate[199]. The PET substrate was laminated on the glass carrier using a cool-off type adhesive during the manufacturing process and delaminated from the glass substrate after the fabrication process was accomplished. 150-nm-thick ITO source/drain electrodes and 20-nm-thick active IGZO channel layer were deposited by sputtering. 100-nm-thick Al2O3 by ALD at 150 °C was used as the gate dielectric. The gate electrode was patterned by Al via thermal evaporation. The saturation mobility, threshold voltage, subthreshold swing, and on/off ratio of the device processed in this way were evaluated as 15.5 cm2V?1s?1, 4.1 V, 0.2 V/decade, and 2.2 × 108, respectively. Fig. 4 shows the parameters of saturation mobility, threshold voltage, and subthreshold swing at different bending curvature radii. The variations of the device parameters under different bending radii can be negligible as shown in Fig. 4.
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Figure4.
(Color online) Device parameters including saturation mobility, threshold voltage, and subthreshold swing versus different bending radii. Reproduced from Ref. [199].
Mativenga et al. deposited a 20-μm-thick polyimide film by spin coating on glass as a compliant substrate for transparent flexible IGZO based TFTs[45]. After all the fabrication processes were completed, the devices were first tested while they were still on the glass carrier. After the test was accomplished, the devices were separated from the glass substrate and attached to a 100-μm-thick PET substrate by a double-sided PI tape. Then a final anneal was performed at 150 °C at vacuum for 2 h. Then the bending properties of the samples were measured. Fig. 5 shows the transfer characteristics before and after detachment from the glass carrier substrate. A slight improvement of the transfer characteristics of the TFT was achieved because of annealing.
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Figure5.
(Color online) Transfer characteristics tested before and after separating from glass carrier to PET. Reproduced from Ref. [45].
Li et al. utilized a polyimide precursor to obtain 4.8-μm-thick polyimide films as a flexible substrate by spin coating on Si wafers[200]. After fabrication, the devices were released from the silicon wafer and formed a free-standing sample. They tested the device transfer characteristics after repeated bending cycles with bending orientations parallel and perpendicular to the current flow direction as shown in Fig. 6. After 50000 bending cycles, the device exhibited a little change above the threshold voltage region, but an increase in the off state current and a negative shift in the turn-on voltage of about 0.3 V. Tripathi et al. tested IGZO-based flexible transistors with PEN substrate[44]. Transfer characteristics were measured while the TFT was bent under different curvature radii with bending orientations parallel and perpendicular to the current flow direction as shown in Fig. 7. As we can see, the slight changes of transfer curves can be negligible under different bending conditions. An extremely small bending curvature radius of about 50 μm was achieved with the assistance of human hair[201, 202].
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Figure6.
(Color online) Transfer characteristics after repeated bending cycles with bending orientations (a) parallel and (b) perpendicular to the current flow direction. The bending radius is about 3.5 mm. Reproduced from Ref. [200].
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Figure7.
(Color online) Transfer characteristics while TFTs bent at different radii (a) parallel and (b) perpendicular to the current flow direction. Reproduced from Ref. [44].
For the field effect transistor, a gate electrode, gate dielectric, and semiconductor compose a capacitor. In order to achieve high performance of the usual gate dielectric, as little movable ions as possible is desired. On the contrary, capacitors based on electric-double-layer (EDL) utilize movable ions in electrolytes to form capacitive effects[203]. A number of materials such as ionic liquids, polymer electrolytes, ionic gels, and inorganic dielectric materials can be used as electrolytes in the EDL capacitor[204, 205]. An apparent advantage of electrolytes is that a large density carrier accumulation can be achieved in the dielectric/semiconductor interface by the EDL capacitor[206]. The specific capacitances of the usual gate dielectrics are of the order of ~10?7 F/cm2 with the induced carrier density of ~1013 cm?2. The specific capacitances of electrolytes are above 10?6 F/cm2 with the accumulation carrier density up to 1015 cm?2[205]. Flexible oxide-based TFTs gated by electrolytes are also widely investigated[207–209].
Junctionless flexible IZO TFTs gated by SiO2-based solid electrolyte on plastic substrate were reported by Zhou et al.[207]. Fig. 8 demonstrates the structure of the device with EDL-based dielectric. The oxide-based TFTs were fabricated on flexible transparent ITO-coated PET substrate. The ITO film was used as the bottom gate electrode. 500-nm-thick phosphorus-doped SiO2-based solid-electrolyte was deposited by plasma enhanced chemical vapor deposition (PECVD) (SiH4/PH3 and O2 as active gases) as the gate dielectric. 30 nm-thick IZO layer arrays (150 ×1000 μm2) were deposited by RF magnetron sputtering. Drain current signals were obtained by contacting two tungsten probes onto the surface of IZO film with a distance (channel length) of 300 μm. The junctionless TFTs exhibited high performance such as a low subthreshold swing of 0.13 V/decade, on/off ratio >10 6, and field-effect mobility of about 60 cm2V?1s?1.
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Figure8.
(Color online) Schematic diagram of the IZO-based junctionless TFT on PET substrate. Reproduced from Ref. [207].
3.2
Paper sheets
Compared with the polymer plastics, paper is a low-cost, renewable, and bio-degradable material in our daily life, which makes it an intriguing alternative substrate for flexible electronics. Eco-friendly paper sheets have attracted increasing attention as compliant substrates for flexible oxide-based TFTs[70, 210–222]. Generally, paper often exhibits rough surface and water absorption. One layer of thin film is usually deposited to prevent paper absorbing water and solvents during the fabrication process and improve the surface roughness. Silicon dioxide deposited by PECVD[223, 224] and epoxy acrylate copolymer layer by coating[225] have been used to improve paper surface roughness.
Lim et al. reported a kind of low-voltage IGZO TFTs on paper substrates[211]. One layer of cyclotene (BCB 3022-35) was deposited on the paper by spin-coating as a water and solvent barrier layer. Thin film transistors processed in this way exhibited saturation mobility of 1.2 cm2V?1s?1, subthreshold swing of 0.65 V/decade, threshold voltage of 1.9 V, and on/off ratio of about 104. Fig. 9 shows the schematic diagram and I–V curve of the TFTs and the surface images with and without BCB coating. We can see that the BCB coating layer improves the surface roughness.
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Figure9.
(Color online) (a) (b) The schematic diagram and I–V curve of the TFTs. (c) (d) The surface images with and without BCB coating. Reproduced from Ref. [211].
Cellulose paper is not only a flexible substrate material but also a good gate dielectric. In 2008, Fortunato et al. reported oxide thin-film field-effect transistors using cellulose-fiber-based paper simultaneously as substrate and gate dielectric for the first time[226]. Fig. 10 illustrates the device configuration. Cellulose-fiber-based paper without any kind of surface treatment was used as substrate carrier and gate dielectric. 40-nm-thick IGZO was deposited by sputtering on one side of the paper sheet as a channel layer. 160-nm-thick IZO were sputtered on the other side of the paper as the gate electrode. The 180-nm-thick aluminum drain and source electrodes were deposited by e-beam evaporation. Transistors fabricated in this way had an enhanced n-type operation mode and exhibited a near zero-voltage threshold voltage, a saturation mobility exceeding 30 cm2V?1s?1, on/off ratio above 104, and a subthreshold swing of about 0.8 V/decade.
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Figure10.
(Color online) The schematic of the flexible oxide-based TFT using cellulose paper as both gate dielectric and substrate carrier. Reproduced from Ref. [226].
In addition, a selective floating gate non-volatile memory transistor, which used cellulose paper as both gate dielectric and substrate was reported by Martins et al.[227]. Fig. 11 demonstrates the selective floating gate non-volatile memory transistor structure. Moreover, Kim et al. reported a nonvolatile memory thin film transistor using biodegradable chicken albumen as the gate dielectric and eco-friendly paper as substrate[43].
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Figure11.
(Color online) Illustration of the device structure with its different layers. The magnified insets show the paper sheet dielectric structure and some of the expected electron paths along the fibers between source and drain electrodes. Reproduced from Ref. [227].
3.3
Metal foils
Metal materials such as gold, silver, copper, and aluminum exhibit excellent mechanical performance and good ductility. Compared with polymer plastics, metal foils exhibit higher thermal resistance and thus widen the processing windows[228]. Ductile metal foils can bend with an extremely small curvature radius so that TFTs with metal foil substrates can be applied to applications which require extremely high strain implementations such as sensors integrated on artificial skin systems[229].
Park et al. demonstrated a kind of flexible thermal and pressure sensors driven by IGZO TFTs fabricated on 50-μm-thick metal foils[229]. Fig. 12 shows a photograph of TFTs fabricated on metal foil under the bending condition. The flexible thermal sensor showed a seven times increase in the output current when the ambient temperature increased from 20 to 100 °C (temperature coefficient of resistance α~2.74%/K) and the pressure sensor also showed a high sensitivity as the pressure varied. The good performance of the sensor fabricated on metal foil confirmed that it can be applied to the artificial skin systems.
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Figure12.
(Color online) Photograph of flexible TFTs with metal foil substrate. Reproduced from Ref. [229].
3.4
Flexible thin glass
In general, glass is chosen as a rigid substrate in electronics. Glass with a thickness of dozens of microns used as a flexible substrate is a very intriguing application[230–233]. Flexible thin glass substrates exhibit a high degree of transparency in nature and can bear the high-temperature annealing process which is important for improving device performance. During the fabrication process of flexible oxide-based TFTs, polyimide substrates are frequently used for high-temperature annealing processes. However, polyimide cannot be applied to transparent displays because of the intrinsic yellowish color of PI itself[127, 234].
Lee et al. reported a kind of high transparent a-IGZO TFTs on a flexible thin glass substrate[231]. The optical transparency of the a-IGZO TFTs fabricated on 70-μm-thick glass substrate was higher than 80% in the visible region from 390 to 750 nm as shown in Fig. 13. The inset in Fig. 13 is a photograph of transparent flexible IGZO TFTs with thin glass substrate under bending state. The bending tests showed that the minimum bending curvature radius was 40 mm. On account of the high temperature annealing process, the device showed very good bias stability under both prolonged positive and negative stress tests.
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Figure13.
(Color online) Optical transmittance versus wavelength of the IGZO TFTs on flexible thin glass, the inset is a photograph of transparent flexible IGZO TFTs fabricated on flexible thin glass under bending status. Reproduced from Ref. [231].
Table 1 shows the bending properties of the flexible oxide-based TFTs. We can see that TFTs based on various oxide active layers and flexible substrates achieved bending properties. The oxide-based TFTs worked well when different strain types including tensile and compressive were applied to them. The flexible oxide-based TFTs operated normally, even the bending radius reached 50 μm or the bending times reached 10 000.
| Active layer | Substrate and thickness (μm) | Bending radius (mm) | Bending times | Strain type | Ref. |
| a-IGZO | PET/200 | 30 | — | Tensile | [30] |
| a-IGZO | PEN/125 | 5 | 10000 | Tensile | [199] |
| ZnO | PI/5 | 3.5 | 5000 | Tensile and compressive | [200] |
| a-IGZO | Parylene/1 | 0.05 | — | Tensile | [201] |
| InOx | PI | 1 | — | — | [182] |
| In2O3 | PI | — | — | — | [77] |
| a-IGZO | PC | 15 | — | — | [166] |
| ZnO | PI/50 | 0.2 | 100 | Tensile | [120] |
| a-IGZO | PI/16 | 10 | 10000 | Tensile | [116] |
| a-IGZO | Paper/52 | 5 | — | Tensile | [220] |
| a-IGZO | PI/20 | 15 | 10000 | — | [178] |
| In2O3 | PEN/— | — | — | — | [83] |
| a-IGO | PI/50 | 10 | — | — | [76] |
| a-IGZO | PEN/125 | 4 | 100 | Tensile | [139] |
| ZnO | PI/50 | 25 | — | Tensile | [208] |
| a-IGZO | PEN/25 | 2 | — | Tensile and compressive | [44] |
| a-IGZO | Thin glass/70 | 40 | — | Tensile | [231] |
| a-IGZO | Al/480 | 19 | — | Tensile | [228] |
Table1.
Bending properties of flexible oxide-based TFTs.
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| Active layer | Substrate and thickness (μm) | Bending radius (mm) | Bending times | Strain type | Ref. |
| a-IGZO | PET/200 | 30 | — | Tensile | [30] |
| a-IGZO | PEN/125 | 5 | 10000 | Tensile | [199] |
| ZnO | PI/5 | 3.5 | 5000 | Tensile and compressive | [200] |
| a-IGZO | Parylene/1 | 0.05 | — | Tensile | [201] |
| InOx | PI | 1 | — | — | [182] |
| In2O3 | PI | — | — | — | [77] |
| a-IGZO | PC | 15 | — | — | [166] |
| ZnO | PI/50 | 0.2 | 100 | Tensile | [120] |
| a-IGZO | PI/16 | 10 | 10000 | Tensile | [116] |
| a-IGZO | Paper/52 | 5 | — | Tensile | [220] |
| a-IGZO | PI/20 | 15 | 10000 | — | [178] |
| In2O3 | PEN/— | — | — | — | [83] |
| a-IGO | PI/50 | 10 | — | — | [76] |
| a-IGZO | PEN/125 | 4 | 100 | Tensile | [139] |
| ZnO | PI/50 | 25 | — | Tensile | [208] |
| a-IGZO | PEN/25 | 2 | — | Tensile and compressive | [44] |
| a-IGZO | Thin glass/70 | 40 | — | Tensile | [231] |
| a-IGZO | Al/480 | 19 | — | Tensile | [228] |
4.
Applications
Flexible oxide-based TFTs have intriguing applications in many areas such as flexible sensors, memories, circuits, and bendable displays.
4.1
Flexible sensors
Flexible sensors have many application areas such as wearable electronics and electronic skin for robots. Low-cost bendable sensors sensing aspects such as pH, X-ray, and temperature achieved by flexible oxide-based TFTs have been demonstrated[40, 41, 142, 145, 235–239].
Shah et al. engineered an integral system composed of an integrated sensor, flexible electronics, and CMOS readout chip[41]. The integrated biosensing system enabled the advantages of the large sensing area provided by flexible oxide-based TFTs process and the information processing capability of CMOS.
Liu et al. reported a flexible oxide-based neuromorphic transistor on PET substrate with multiple input gates for pH sensing[40]. Sensitivity of about 105 mV/pH was obtained when the device worked in a quasi-static dual-gate synergic mode. The single-spike sensing mode which was inspired by the spiking operation mode of the biological neuron was demonstrated to enhance the pH sensitivity and reduce the response/recover time and the power dissipation. The tests showed that the single-spike sensing mode could tremendously reduce the power consumption because the device was usually biased at zero voltage and low voltage spikes with very short duration time were applied for one measurement.
Smith et al. presented a flexible digital X-ray detector constructed with an active-matrix array of imaging pixels[236]. Each pixel consisted of one IGZO-based TFT and a p-i-n photodiode. A key medical imaging application of the transparent digital X-ray detector was single-exposure, low-dose, and full-body digital radiography. Fig. 14 shows the flexible electronic tiles for large area digital X-ray imaging.
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Figure14.
(Color online) Flexible electronic tiles for large area digital X-ray imaging. Reproduced from Ref. [236].
4.2
Flexible memories
Flexible electronic systems require memories with low-power consumption and stable bending performance. Recently, flexible memory transistors based on oxide TFTs have attracted increasing attention[42, 124, 144, 146, 149, 240].
A flexible charge-trap-type IGZO memory TFT fabricated on PEN substrate was proposed by Kim et al.[146]. Zinc oxide and aluminium oxide layers were used as charge-trap layer and blocking/tunneling layers, respectively. A wide memory margin (25.6 V), programming speed of ~500 ns, and long retention time of about 3 h were achieved at room temperature and 80 °C. The transfer characteristics and on/off programming operations of the memory device did not show marked degradation even after 104 bending events at a curvature radius of 4.8 mm.
A p-type SnO-based polymer ferroelectric field-effect memory device on polyimide substrate was proposed by Caraveo-Frescas et al.[100]. Fig. 15 shows an optical profilometer scan of the TFT memory showing major parts of the device. The p-type SnO-based TFT memory exhibited a field effect mobility of 2.5 cm2V?1s?1 and a high memory window of 16 V.
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Figure15.
(Color online) Optical profilometer scan of the SnO-based polymer ferroelectric TFT memory. Reproduced from Ref. [100].
4.3
Flexible circuits
Higher integration level of TFTs is requisite for manufacturing flexible circuits for future applications such as RFID[18], transponder chip[241], and scan driver[242]. However, most TFT technologies have only monotype devices. Flexible circuits achieved by oxide-based TFTs such as ring oscillator, clock generator, operational amplifier (opamp), and transponder chip were reported[44, 45, 117, 119, 130, 182, 243–247].
Huang et al. reported a design style named pseudo-CMOS for low-cost and robust flexible electronics using monotype single-Vth or dual-Vth TFTs[244]. To validate the design style, they fabricated digital cells in two different TFT technologies, i.e., p-type self-assembled-monolayer-organic TFTs and n-type IGZO-based TFTs. A compact design model for flexible analog/RF circuits with a-IGZO TFTs was also presented by Perumal et al.[248].
Zhao et al. fabricated a flexible 15-stage ring oscillator using ZnO TFTs on polyimide substrate[130]. The oscillator was operated at greater than 2 MHz with a supply voltage of 18 V and the propagation delay was less than 20 ns/stage. Fig. 16 shows flexible ZnO circuits on plastic substrate.
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Figure16.
(Color online) (Left) Optical microscopic image of ZnO TFT. (Top right) Schematic diagram of ZnO TFT and (bottom right) ZnO TFTs and circuits on flexible plastic substrate. Reproduced from Ref. [130].
A full-swing flexible clock generator based on a-IGZO dual-gate TFTs was reported by Chen et al.[119]. Clock signals were generated by a 13-stage ring oscillator, which was achieved by n-type-only dual-gate a-IGZO TFTs. Output frequency of 360 kHz was achieved by combining bootstrapping and pseudo-CMOS inverting.
An 8-b RFID transponder chip with an integration level of ~300 was manufactured with IGZO-based TFTs on PEN substrate by Tripathi et al.[44]. The transponder chip obtained a data transfer rate of 31.6 kb/s when operated under bending condition down to a bending radius of 2 mm. It took ~253 μs to generate the 8-b code with the supply voltage of 10 V.
Simulation and fabrication study of flexible IGZO-based circuits with two and three metal layers were reported by Cantarella et al.[117]. They fabricated flexible opamps and logic circuits based on IGZO TFTs under the guidance of systematic computer-aided design simulations. The opamps exhibited a gain of 19.4 dB, cut-off frequency of 7 kHz, and gain-bandwidth-product (GBWP) of 40 kHz, and the results showed good agreement with the simulation results.
Complementary inverters are expected for circuit integrations. Flexible complementary circuits based on both p- and n-channel oxide TFT technologies[2, 46, 70] and hybrid organic/ inorganic TFT technologies have been studied[169, 170, 249–252]. A flexible ring oscillator achieved by complementary circuits employing n-type ZnO and p-type SnO TFTs was reported by Li et al.[2]. Hybrid complementary inverters composed of p-type pentacene and n-type a-IGZO TFTs with PES substrate were proposed by Kim et al.[169].
4.4
Flexible displays
In recent years, amorphous oxide semiconductors, especially a-IGZO based TFTs have received a great deal of attention in the display backplanes due to high electron mobility, low process temperature, and good uniformity. Amorphous oxide-based TFTs fabricated on flexible substrates are promising alternatives for manufacturing bendable active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting diode (AMOLED) displays[8, 47–53, 148, 253–258].
In 2009, Park et al. fabricated a 6.5 in. flexible full color organic light-emitting diode display on polyimide substrate driven by a-IGZO TFTs[47]. The a-IGZO based TFTs exhibited field-effect mobility exceeding 15.1 cm2V?1s?1, subthreshold swing of 250 mV/decade, and threshold voltage of 0.9 V. Thin film transistors with 10-μm-thick polyimide substrate underwent bending down to R = 3 mm under tension and compression without any performance degradation. Fig. 17 shows the 6.5 in. flexible full-color top-emission AMOLED panel driven by a-IGZO TFTs. The displayed image did not show any electrical performance degradation when the panel was bent to about 2 cm of curvature. The total thickness of the AMOLED display was less than 0.1 mm.
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Figure17.
(Color online) Picture displayed by 6.5 in. flexible full-color top-emission AMOLED panel bent to a curvature radius of approximately 2 cm. Reproduced from Ref. [47].
Table 2 lists a portion of the recent studies of flexible oxide-based TFTs in different application areas. High mobility，high on/off ratio, and low subthreshold swing were achieved in flexible oxide-based TFTs. Besides displays, the oxide-based TFTs could also be applied to sensors, memories, and circuits. Sensors and memories based on flexible oxide TFTs have potential applications on flexible systems such as wearable electronics. Complementary circuits based on both n- and p-channel oxide TFTs can be applied to flexible low-power consumption electronic systems.
| Active layer | Dielectric | Substrate | μFE (cm2V?1s?1) | Ion/Ioff | Vth(V) | SS (V/decade) | Application | Ref. |
| IZO | SiO2 | PET | 12 | 6.4 × 105 | ?0.3 | 0.175 | PH sensor | [40] |
| a-IGZO | SiO-based | PEN | 10–15 | — | 2 | 0.3 | X-ray detector | [145] |
| ZnO | Ferroelectric copolymer | PEN | 33.3 | 108 | — | 0.65 | Memory | [144] |
| a-IGZO | Ferroelectric copolymer | PEN | 1 | 107 | — | 0.4 | Memory | [149] |
| SnO | Poly ferroelectric | PI | 2.53 | 0.94 × 102 | ?11.7 | 4.35 | Memory | [100] |
| SnOx/a-IGZO | Paper | Paper | 1.3/23 | 104/102 | 2.1/1.4 | 3.1/6.9 | Complementary circuits | [70] |
| SnO/ZnO | HfO2 | PI | 0.06/1.6 | 104/106 | 5/5 | 1.6/1.6 | Ring oscillator | [2] |
| ZnO | Al2O3 | PI | 20 | 107 | 2 | 0.35 | Ring oscillator | [130] |
| a-IGZO | SiNx | PEN | 13 | 108 | 3.1 | 0.33 | 8-b transponder chip | [44] |
| InOx | Al2O3 | PI | 8.02 | — | 3.67 | — | Ring oscillator | [182] |
| a-IGZO | SiO2 | PI | 17 | — | 1.4 | — | Clock generating circuit | [119] |
| ITZO | SiO2 | PI | 32.9 | — | — | — | iOLED | [48] |
| a-IGZO | AlOx | PEN | 12.87 | 109 | 2.48 | 0.20 | AMOLED | [8] |
| a-IGZO | SiNx | PI | 15.1 | 5 × 108 | 0.9 | 0.25 | AMOLED | [47] |
| a-IGZO | Al2O3 | PEN | 11.2 | 109 | 0.5 | 0.27 | AMOLED | [52] |
Table2.
Applications of flexible oxide-based thin film transistors.
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| Active layer | Dielectric | Substrate | μFE (cm2V?1s?1) | Ion/Ioff | Vth(V) | SS (V/decade) | Application | Ref. |
| IZO | SiO2 | PET | 12 | 6.4 × 105 | ?0.3 | 0.175 | PH sensor | [40] |
| a-IGZO | SiO-based | PEN | 10–15 | — | 2 | 0.3 | X-ray detector | [145] |
| ZnO | Ferroelectric copolymer | PEN | 33.3 | 108 | — | 0.65 | Memory | [144] |
| a-IGZO | Ferroelectric copolymer | PEN | 1 | 107 | — | 0.4 | Memory | [149] |
| SnO | Poly ferroelectric | PI | 2.53 | 0.94 × 102 | ?11.7 | 4.35 | Memory | [100] |
| SnOx/a-IGZO | Paper | Paper | 1.3/23 | 104/102 | 2.1/1.4 | 3.1/6.9 | Complementary circuits | [70] |
| SnO/ZnO | HfO2 | PI | 0.06/1.6 | 104/106 | 5/5 | 1.6/1.6 | Ring oscillator | [2] |
| ZnO | Al2O3 | PI | 20 | 107 | 2 | 0.35 | Ring oscillator | [130] |
| a-IGZO | SiNx | PEN | 13 | 108 | 3.1 | 0.33 | 8-b transponder chip | [44] |
| InOx | Al2O3 | PI | 8.02 | — | 3.67 | — | Ring oscillator | [182] |
| a-IGZO | SiO2 | PI | 17 | — | 1.4 | — | Clock generating circuit | [119] |
| ITZO | SiO2 | PI | 32.9 | — | — | — | iOLED | [48] |
| a-IGZO | AlOx | PEN | 12.87 | 109 | 2.48 | 0.20 | AMOLED | [8] |
| a-IGZO | SiNx | PI | 15.1 | 5 × 108 | 0.9 | 0.25 | AMOLED | [47] |
| a-IGZO | Al2O3 | PEN | 11.2 | 109 | 0.5 | 0.27 | AMOLED | [52] |
5.
Conclusions and outlook
We have presented a review of the progress of flexible oxide-based thin film transistors. Flexible oxide-based TFTs were introduced classified by compliant substrates including polymer plastics, paper sheets, metal foils, and flexible thin glass. We can see that flexible oxide-based TFTs can work normally, even undergoing high strain implementation and repeatable bending events, which confirm that oxide-based TFTs are suitable for flexible electronic platforms. Flexible oxide-based TFTs not only are promising candidates for future bendable displays but also have many application areas such as sensors, memories, and circuits.
In the future, with the development of flexible oxide-based TFTs, displays will exhibit better bending properties, higher resolution, larger size, thinner thickness, or even better transparency. Various flexible sensors based on oxide TFTs will achieve multiple monitoring functions such as pH, temperature, humidity, blood sugar and pressure, and heart rate. With the development of high performance p-type oxide semiconductor materials, flexible circuits based on both p- and n-type oxide semiconductor TFTs will derive rapid development.
