Low-Power CMOS-like Flexible Circuits With Unipolar TFTs

Abstract

The increasing demand for Thin Film Transistor (TFT) electronics is driven by their cost effectiveness and suitability for high-volume production, making them ideal for applications in flexible electronics, wearable devices, on-body sensors, and Internet-of things (IoT) devices. TFT technologies, including organic, transition metal oxide (TMO), and amorphous silicon (a-Si:H) TFTs, have demonstrated significant potential for large-area, low-cost fabrication. Their integration into flexible electronics has enabled innovations such as bendable displays, electronic skin, and smart patches. However, unlike Complementary Metal-Oxide-Semiconductor (CMOS) technology, TFTs lack complementary transistors. Therefore, most complex circuits on flexible substrates remain reliant on rigid CMOS integrated circuits (IC)s, connected via soft ribbon cables. This dependency introduces challenges in signal interfacing, cost, and reliability, which constrain the scalability and complexity of flexible electronics. In display technology, TFTs are predominantly used for pixel circuits, while off-panel CMOS ICs handle control and driver circuits. Particularly, for high resolution displays, the need for numerous bonding pads to interface with off-panel ICs creates bottlenecks due to mechanical pitch constraints, and limited scalability, and exacerbates power dissipation caused by high-capacitance bonding pads. Addressing these challenges requires integrating control logic and driver circuits directly onto the TFT backplane to minimize reliance on off-panel CMOS circuits. Unipolar TFT logic circuits face challenges such as limited output swing and significant direct path current, further complicating their use in low-power applications. To overcome these limitations, various fabrication techniques have been explored to create complementary transistors, but their higher production costs and complexities have limited practical adoption. While several circuit designs have been proposed to achieve full output swing using unipolar TFTs, the persistent issue of substantial direct path current remains a critical obstacle. Moreover, most existing studies fail to consider the impact of bending on device power consumption and performance. This thesis presents a methodology for designing low-power, full-swing TFT digital circuits, including logic gates, decoders, D Flip-Flops (DFF), and Static Random Access Memory (SRAM) memory cells. The proposed circuits are fabricated on both glass and flexible polyethylene naphthalate (PEN) substrates, with an in-depth analysis of substrate bending effects on device performance. This study paves the way for enhancing existing TFT models by incorporating bending effects. The designs achieve significant advancements, including over 46.2% power reduction compared to state-of-the-art 3-to-8 address decoder design. These contributions lay the groundwork for low-power “system-on-flex” applications, providing a path toward scalable, efficient, and integrated flexible electronics.