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

Abstract

T hin-film transistors (TFTs) are currently the dominant technology for in-pixel switches and drivers in flat-panel displays. Trends in consumer electronics demand ever-higher display resolution and brightness, lower power consumption, and new features and form factors (such as curved and foldable displays). This drives TFT devices to deliver more complex functions than simply switching. For example, recent bezel-less displays delegate the task of row selection to TFT circuits integrated next to the pixel array. Such driver circuits comprise thousands of switches operating together -previously a job for silicon chips mounted around the display.Beyond displays, how far can thin-film circuits go in terms of replacing silicon complementary metal-oxide-semiconductor (CMOS) chips? Fig. 1 illustrates the key advantages of thin-film circuits on flexible substrates. The circuits can be fabricated on large substrates, thereby creating very thin, lightweight and ultra-flexible electronics 1,2 . Folding, rolling or crumpling such flexible circuits is possible without destroying the electronic functionality. Because of these properties, a flexible TFT-based microprocessor 3 (Fig. 1d) or thin-film near-field communication (NFC) tag (Fig. 1e) can, for example, be integrated imperceptibly into any object. TFT technologies also have considerable potential for fabrication on ultrathin stretchable substrates 4 that can be made porous to create breathable devices for contact with skin. With these features, thin-film integrated circuits (ICs) could be a game-changer for wearable electronics. Ultrathin, conformable ICs in the form of a tattoo could be used to monitor vital body parameters, communicating these parameters directly to a patient's smartphone or a medical database.In this Perspective, I discuss the potential of thin-film circuits on plastic substrates for the development of Internet of Things (IoT) and wearable applications. First I look at the different TFT technologies that can be realized on flexible substrates, and then discuss the impact TFT technology will have on circuit design at the level of digital logic gates and very-large-scale integration (VLSI) digital circuits. For the latter, I consider the use of thin-film ICs in the creation of low-cost radiofrequency identification (RFID) tags for everyday items. Flexible chips may initially be used to simply identify each item. Then, when the RFID chip is potentially replaced with a thinfilm NFC chip, standard smartphones and tablets equipped with NFC readers could identify objects and connect them to the cloud. Another advantage of thin-film IC technology is its potential to be combined with sensor or signage technology, thereby integrating more functionality into the objects, which is discussed in a section on analogue circuits. Finally, I will briefly examine the potential of silicon CMOS chip technology to be interfaced directly with TFT circuitry. TFT technologyThe development and optimization of transistor technologies are driven by four important figures of me...

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