The field of flexible electronics has rapidly expanded over the last decades, pioneering novel applications, such as wearable and textile integrated devices, seamless and embedded patch-like systems, soft electronic skins, as well as imperceptible and transient implants. The possibility to revolutionize our daily life with such disruptive appliances has fueled the quest for electronic devices which yield good electrical and mechanical performance and are at the same time light-weight, transparent, conformable, stretchable, and even biodegradable. Flexible metal oxide semiconductor thin-film transistors (TFTs) can fulfill all these requirements and are therefore considered the most promising technology for tomorrow's electronics. This review reflects the establishment of flexible metal oxide semiconductor TFTs, from the development of single devices, large-area circuits, up to entirely integrated systems. First, an introduction on metal oxide semiconductor TFTs is given, where the history of the field is revisited, the TFT configurations and operating principles are presented, and the main issues and technological challenges faced in the area are analyzed. Then, the recent advances achieved for flexible n-type metal oxide semiconductor TFTs manufactured by physical vapor deposition methods and solution-processing techniques are summarized. In particular, the ability of flexible metal oxide semiconductor TFTs to combine low temperature fabrication, high carrier mobility, large frequency operation, extreme mechanical bendability, together with transparency, conformability, stretchability, and water dissolubility is shown. Afterward, a detailed analysis of the most promising metal oxide semiconducting materials developed to realize the state-of-the-art flexible p-type TFTs is given. Next, the recent progresses obtained for flexible metal oxide semiconductor-based electronic circuits, realized with both unipolar and complementary technology, are reported. In particular, the realization of large-area digital circuitry like flexible near field communication tags and analog integrated circuits such as bendable operational amplifiers is presented. The last topic of this review is devoted for emerging flexible electronic systems, from foldable displays, power transmission elements to integrated systems for large-area sensing and data storage and transmission. Finally, the conclusions are drawn and an outlook over the field with a prediction for the future is provided.

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Metal oxide semiconductor thin-film transistors for flexible electronics

Organic thin film transistor (OTFT) based device modeling and circuit application is a rapidly emerging research area. Taking cognizance of this fact, our paper reviews various basic to advanced OTFT structures, their performance parameters, materials of individual OTFT layers, their molecular structures, OTFT charge transport phenomena, and fabrication techniques. The performance of p- and n-type conducting polymer and small molecule organic semiconductors are reviewed primarily in terms of field effect mobility, current on/off ratio, and operating voltage for various OTFT structures. Moreover, different organic/inorganic materials for realizing the dielectric layer, electrodes, and the substrate in an OTFT are analyzed. Some of the compact models that are essential for predicting and optimizing the device performance are described that takes into account the mobility enhancement factor and channel length modulation. A detailed study of the single gate, dual gate, vertical channel, and cylindrical gate O...

Organic Thin Film Transistors: Structures, Models, Materials, Fabrication, and Applications: A Review

The often touted attractive attributes of printed/organic electronics are its mechanically flexible form-factor, low-cost, green, on-demand printing, scalability, low-power operation, and intelligence (signal processing) – ideally, the creation of intelligent lightweight electronics printed by simple ubiquitous printing processes, and integrated into new ways to exploit its mechanically flexible form-factor. Printed/Organic Electronics, now an industry on its own right and recognized as one of the key technological enablers for the Internet of Things, is largely complementary to silicon because the printed transistors are slow and the printed elements are large. The sanguine projected growth of the $29 B market today to $73 B by 2027 assumes that ‘intelligence’ (analog, mixed-signal and digital signal processing) would be realizable. Nevertheless, many of the said attributes of printed/organic electronics remain a challenge. In this paper, we exemplify this with a comprehensive and critical review and tabulation of the state-of-the art printed digital, analog, and mixed-signal circuits. We further review the application space of printed/organic electronics and the supply chain, including their classifications and delineate the associated challenges in each constituent chain. These challenges, largely unresolved, are indeed formidable, and are discussed with a critical circuits and systems perspective. Our review depicts that contemporary design philosophies and methodologies for silicon are largely inadequate for printed/organic electronics. To this end, we discuss esoteric analog and digital design philosophies and methodologies, with emphasis on co-design and co-optimization between the different constituent supply chains that may potentially circumvent the said formidable challenges, and discuss the associated penalties thereto.

A Circuits and Systems Perspective of Organic/Printed Electronics: Review, Challenges, and Contemporary and Emerging Design Approaches

Attributed to its advantages of super mechanical flexibility, very low-temperature processing, and compatibility with low cost and high throughput manufacturing, organic thin-film transistor (OTFT) technology is able to bring electrical, mechanical, and industrial benefits to a wide range of new applications by activating nonflat surfaces with flexible displays, sensors, and other electronic functions. Despite both strong application demand and these significant technological advances, there is still a gap to be filled for OTFT technology to be widely commercially adopted. This paper provides a comprehensive review of the current status of OTFT technologies ranging from material, device, process, and integration, to design and system applications, and clarifies the real challenges behind to be addressed.

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Current Status and Opportunities of Organic Thin-Film Transistor Technologies

Forty years after the first silicon microprocessors, we demonstrate an 8-bit microprocessor made from plastic electronic technology directly on flexible plastic foil. The operation speed is today limited to 40 instructions per second. The power consumption is as low as 100 μW. The ALU-foil operates at a supply voltage of 10 V and back-gate voltage of 50 V. The microprocessor can execute user-defined programs: we demonstrate the execution of the multiplication of two 4-bit numbers and the calculation of the moving average of a string of incoming 6-bit numbers. To execute such dedicated tasks on the microprocessor, we create small plastic circuits that generate the sequences of appropriate instructions. The near transparency, mechanical flexibility, and low power consumption of the processor are attractive features for integration on everyday objects, where it could be programmed as, amongst other items, a calculator, timer, or game controller.

An 8-Bit, 40-Instructions-Per-Second Organic Microprocessor on Plastic Foil

In this work we present a fully integrated first order continuous-time ΔΣ ADC made in a pentacene-based dual-gate organic thin-film transistor technology on flexible plastic foil. The ADC achieves a precision of 26.5 dB at a clock speed of 500 Hz and draws 100 μA from a 15 V power supply. As sub-blocks of the ADC, we also present a Vt -insensitive single-stage differential amplifier with 10 kHz GBW, a 3-stage operational amplifier, an integrator, a comparator and a level shifter. The circuits are designed following a strict Vt -insensitive design strategy and use high-pass filters for offset cancellation. The active area is 13 × 20 mm2.

A Fully Integrated $\Delta \Sigma$ ADC in Organic Thin-Film Transistor Technology on Flexible Plastic Foil

In this letter, a compact model for the dc current and capacitance of organic thin-film transistors is proposed. This model is based on the variable-range hopping-transport theory, and the double-exponential density-of-state function is assumed. The presented model can accurately describe the current from subthreshold regime to linear and saturation regime via a single unified formulation. Good agreement between the theoretical calculation and measurements of pentacene field-effect transistor is found. The application of this compact model on circuit simulation is also presented.

Compact Model for Organic Thin-Film Transistor

In this work, we present the implementation and measurement results of all analog building blocks of an organic smart sensor system on foil. The presented building blocks are a 1D and a 2D 4 × 4 pixel flexible capacitive touch sensor with a sample rate of 1.5 kS/s, a DC-connected two-stage opamp with a 20 dB DC gain, a Dickson DC-DC up-converter with output bias voltages up to 60 V and down to - 40 V which are used as a bias voltage in the other building blocks, and a ΔΣ ADC with a 26.5 dB precision and a band width of 15.6 Hz. The sensors, the opamp, the DC-DC converter and the ADC respectively consume 6 μA, 15 μA, 1 μA and 100 μA from a 15 V power supply.

Analog Building Blocks for Organic Smart Sensor Systems in Organic Thin-Film Transistor Technology on Flexible Plastic Foil

Analog techniques for reliable organic circuit design on foil applied to an 18 dB single-stage differential amplifier

Organic electronics is expected to find commercial applications in flexible displays, RFID tags and smart sensor systems, e.g. for food industry or biomedical applications. Key benefits of the technology are the direct production of transistors and circuits on flexible plastic foils, the possibility to directly integrate sensors, light sources, light detectors, a.o. with the same technology, and the low processing temperatures that warrant cost-efficient production. However, organic electronics technologies suffer from important drawbacks versus silicon based technologies, such as its intrinsically lower mobility, the large parameter variation and a very low intrinsic transistor gain (typically 5). Moreover as active components almost exclusively p-type transistors are available and as passive components only capacitors exist. In place of resistors, we are limited to only linear biased transistors. Work on organic RFID [1,2] and several types of organic sensors [3] has been presented. Analog designs in organic technology are in their infancy: a first differential amplifier with differential-mode gain of 10 was presented in [4]; design considerations for analog designs were discussed in [5]; a comparator was presented in [6]; and a 6-bit D/A converter based on a C-2C chain in [7]. In the present work, we disclose the first ADC designed, fabricated and measured in an organic technology on plastic foil with a fully analog design approach.

Hagen Marien

Papers

A Fully Integrated $\Delta \Sigma$ ADC in Organic Thin-Film Transistor Technology on Flexible Plastic Foil

Compact Model for Organic Thin-Film Transistor

Analog Building Blocks for Organic Smart Sensor Systems in Organic Thin-Film Transistor Technology on Flexible Plastic Foil

Analog techniques for reliable organic circuit design on foil applied to an 18 dB single-stage differential amplifier

An analog organic first-order CT ΔΣ ADC on a flexible plastic substrate with 26.5dB precision