The use of thin-film transistors in liquid-crystal display applications was commercialized about 30 years ago. The key advantages of thin-film transistor technologies compared with traditional silicon complementary metal–oxide–semiconductor(CMOS) transistors are their ability to be manufactured on large substrates at low-cost per unit area and at low processing temperatures, which allows them to be directly integrated onto a variety of flexible substrates. Here, I discuss the potential of thin-film transistor technologies in the development of low-cost, flexible integrated circuits for applications beyond flat-panel displays, including the Internet of Things and lightweight wearable electronics. Focusing on the relatively mature thin-film transistor technologies that are available in semiconductor fabrication plants today, the different technologies are evaluated in terms of their potential circuit applications and the implications they will have in the design of integrated circuits, from basic logic gates to more complex digital and analogue systems. I also discuss microprocessors and non-silicon, near-field communication tags that can communicate with smartphones, and I propose the concept of a Moore’s law for flexible electronics. This Perspective discusses the potential of thin-film transistor technologies in the development of low-cost, flexible integrated circuits, evaluating the more mature technologies available today in terms of their potential circuit applications and the implications they will have in the design of integrated circuits.

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The development of flexible integrated circuits based on thin-film transistors

Amorphous InGaZnOx (a-IGZO) thin-film transistors (TFTs) are currently used in flat-panel displays due to their beneficial properties. However, the mobility of ∼10 cm2/(V s) for the a-IGZO TFTs used in commercial organic light-emitting diode TVs is not satisfactory for high-resolution display applications such as virtual and augmented reality applications. In general, the electrical properties of amorphous oxide semiconductors are strongly dependent on their chemical composition; the indium (In)-rich IGZO achieves a high mobility of 50 cm2/(V s). However, the In-rich IGZO TFTs possess another issue of negative threshold voltage owing to intrinsically high carrier density. Therefore, the development of an effective way of carrier density suppression in In-rich IGZO will be a key strategy to the realization of practical high-mobility a-IGZO TFTs. In this study, we report that In-rich IGZO TFTs with vertically stacked InOx, ZnOx, and GaOx atomic layers exhibit excellent performances such as saturation mobilities of ∼74 cm2/(V s), threshold voltage of -1.3 V, on/off ratio of 8.9 × 108, subthreshold swing of 0.26 V/decade, and hysteresis of 0.2 V, while keeping a reasonable carrier density of ∼1017 cm-3. We found that the vertical dimension control of IGZO active layers is critical to TFT performance parameters such as mobility and threshold voltage. This study illustrates the potential advantages of atomic layer deposition processes for fabricating ultrahigh-mobility oxide TFTs.

Amorphous IGZO TFT with High Mobility of ∼70 cm2/(V s) via Vertical Dimension Control Using PEALD

The excited holes occupying the valence band tail states in amorphous oxide semiconductors are found to induce formation of meta-stable O$_2^{2-}$ peroxide defects. The valence band tail states are at least partly characterized by the O-O pp{\sigma}* molecular orbital, and the localized-hole-mediated lattice instability results in the formation of the peroxide defects. Along with the O-O bond formation, the pp{\sigma}* state is heightened up into the conduction bands, and two electrons are accordingly doped in the electronic ground state. The energy barrier from the O$_2^{2-}$ peroxide state to the normal disorder state is found to be 0.97 eV in hybrid density functional theory. The hole-mediated formation of the meta-stable peroxide defects and their meta-stability is suggested as an origin of the negative bias and/or illumination stress instability in amorphous oxide semiconductors.

Instability of Amorphous Oxide Semiconductors via Carrier-Mediated Structural Transition between Disorder and Peroxide State

Recent trends in thin film encapsulations (TFEs), fabricating organic/inorganic encapsulation films are reviewed. Atomic layer deposited inorganic films have superior barrier performance and have advantages of excellent uniformity over large scales at relatively low deposition temperatures. However, organic film should be combined with a hybrid structure for improved flexibility and longer lag time. We introduce various organic deposition methods and mechanisms for enhancing barrier performance. However, stress engineering is required to achieve high performance TFEs for flexible devices, new materials and deposition methods.
 First, modulating the internal stress in TFEs should be considered to increase flexibility by adopting other layers that can reduce internal stress. Second, controlling the configuration of the hybrid structure can prevent degradation due to cracks. Third, the introduction of a neutral plane as the middle layer decreases the strain. The results summarize how the device can improve barrier performance under external stress. This paper can guide the improvements in barrier performance.

Review of Organic/Inorganic Thin Film Encapsulation by Atomic Layer Deposition for a Flexible OLED Display

Controlling the anisotropy of two-dimensional materials with orientation-dependent heat transfer characteristics is a possible solution to resolve severe thermal issues in future electronic devices We demonstrate a dramatic enhancement in the in-plane thermal conductivity of stretchable poly(vinyl alcohol) (PVA) nanohybrid films containing small amounts (below 10 wt %) of hexagonal boron nitride ( h-BN) nanoplatelets The h-BN nanoplatelets were homogeneously dispersed in the PVA polymer solution by ultrasonication without additional surface modification The mixture was used to prepare thermally conductive nanocomposite films The in-plane thermal conductivity of the resulting PVA/ h-BN nanocomposite films increased to 64 W/mK when the strain was increased from 0 to 100% in the horizontal direction More specifically, the thermal conductivity of a PVA/ h-BN composite film with 10 wt % filler loading can be improved by up to 32 times as compared to pristine PVA This outstanding thermal conductivity value is significantly larger than that of materials currently used in in-plane thermal management systems This result is attributed to the anisotropic alignment of h-BN particles in the PVA chain matrix during stretching, enhancing phonon conductive paths and hence improving the thermal conductivity and thermal properties of PVA/ h-BN nanocomposite films These polymer nanocomposites have low cost as the amount of expensive conductive fillers is reduced and can be potentially used as high-performance materials for thermal management systems such as heat sink and thermal interface materials, for future electronic and electrical devices

Anisotropy-Driven High Thermal Conductivity in Stretchable Poly(vinyl alcohol)/Hexagonal Boron Nitride Nanohybrid Films.

This paper describes the recent advances in flexible oxide thin-film transistors (TFTs), one of the rapidly emerging technologies for the next-generation display applications. First, the paper focu...

https://tandfonline.com/doi/pdf/10.1080/15980316.2017.1385544

Review of recent advances in flexible oxide semiconductor thin-film transistors

We investigated the effects of repetitive mechanical bending stress on top-gate amorphous InGaZnO thin-film transistors (TFTs). Electrical parameters were gradually degraded under repetitive tensile bending stress. After 50 000 bending cycles, some TFTs showed gate leakage current increase during positive gate bias thermal stress. After 60 000 bending cycles, conduction path was physically severed to an open state. However, when an additional organic layer was deposited on the TFTs as a stress-reduction layer, device characteristics were unaffected by repetitive mechanical stress up to 100 000 cycles. Finite element structural simulations show the vulnerable stress-concentrated regions that cause leakage current, contact resistance increase, and interface traps. Electrical deterioration under repetitive bending is significantly mitigated by applying a stress-reduction layer.

Effects of Repetitive Mechanical Stress on Flexible Oxide Thin-Film Transistors and Stress Reduction via Additional Organic Layer

Previously reported thin-film transistor (TFT) digital logic gates are mostly static circuits. If the static logic circuits are implemented using either nMOS or pMOS technologies alone, unlike CMOS technologies, the circuits consume high power because of the steady-state current, and take large circuit area. In this paper, the dynamic logic circuits using n-type a-IGZO TFTs are proposed to resolve the power and circuit area issues. The dynamic logic circuits such as inverters and nand gates are fabricated in an amorphous indium–gallium–zinc–oxide TFT technology, and traditional static logic circuits are also implemented with the same technology for comparison purposes. The measurement results show that the proposed dynamic logic circuit consumes no steady-state current, and the circuit area is reduced by 93.1%.

Dynamic Logic Circuits Using a-IGZO TFTs

Most thin-film transistor (TFT) gate drivers integrated on the display panel use the carry signals between the stages. In our previous works, we found that these carry-type gate drivers are subject to the reliability problem in a flexible display, since the errors of the stressed stages are accumulated through the carry signals. This problem possibly leads to the failure of image refresh of the display device. Therefore, the carry-free gate driver can resolve the error accumulation problem since it does not use the carry signals and instead, each unit stage operates independently. In this paper, we propose an advanced carry-free gate driver circuit to remove the drawbacks of the previous version, while keeping the advantages. More importantly, this paper is the first report to experimentally show that the error accumulation phenomenon of the carry-type gate driver and the problem is removed in the carry-free gate driver. The proposed carry-free gate driver and the carry-type gate driver are fabricated with amorphous indium-gallium-zinc-oxide TFTs on a flexible substrate for comparison purpose. The proposed gate driver shows a very high reliability since no voltage fluctuation occurs at the outputs after 10 000 cycles bending test with 2-mm bending radius.

A High-Reliability Carry-Free Gate Driver for Flexible Displays Using a-IGZO TFTs

We examine the effects of repetitive rolling stress on thin-film transistors (TFTs) using various rolling radii, and evaluate the subsequent thermal treatment at various temperatures. After applying repetitive rolling stress, the electrical characteristics of the TFTs changes. In particular, change of subthreshold swing (SS) is significant than that of threshold voltage (Vth), and saturation mobility ( $\mu _{\text {sat}}$ ). We simulated the electrical performance of TFTs using technology computer-aided design (TCAD) to quantitatively ascertain the change to donor-like and acceptor-like defects in the active region. Both Gaussian donor-like defects (NGD) and acceptor-like defects (NGA) increased under mechanical strain due to the formation of oxygen-related defects, oxygen vacancy (Vo), and oxygen interstitials (Oi). In addition, negative bias illumination stress (NBIS) reliability results show that degraded TFTs at higher rolling stress yield more variation of threshold voltage ( $\Delta \text{V}_{\text {th}}$ ) and $\Delta $ SS due to increase in interface trap site, and defect sites. The rolling stress generates more oxygen-related defects, Vo and Oi, which act as TFT degradation factors. Finally, we investigate the possible recovery mechanism of mechanically degraded TFTs through thermal treatment. Thermal treatment at 150 °C and 250 °C cannot recover the electrical performance of mechanically stressed TFTs and NBIS reliability to before rolling state. Therefore, we used TCAD simulation to speculate the origin of incomplete recovery. We thought that this incomplete recovery originated from oxygen dimers (O-O) formed from Oi under mechanical stress.

Ki-Lim Han

Papers

Review of recent advances in flexible oxide semiconductor thin-film transistors

Effects of Repetitive Mechanical Stress on Flexible Oxide Thin-Film Transistors and Stress Reduction via Additional Organic Layer

Dynamic Logic Circuits Using a-IGZO TFTs

A High-Reliability Carry-Free Gate Driver for Flexible Displays Using a-IGZO TFTs

Soft Recovery Process of Mechanically Degraded Flexible a-IGZO TFTs With Various Rolling Stresses and Defect Simulation Using TCAD Simulation