Engineering of Thin-Film Transistors for Chemical Sensing

Abstract

Semiconductor technologies underpin modern integrated circuit design. Among them, thin-film transistors (TFTs) play an increasingly significant role in emerging applications such as edge computing, the Internet of Things and healthcare. TFTs are especially valued in large-area electronics for their low-cost fabrication, straightforward processing and scalability with high device uniformity. They have also gained attention as next-generation, high-throughput chemiresistive gas sensors, with applications in medical diagnostics, defence and environmental monitoring. A notable example is their use in exhaled breath analysis.

This thesis focuses on fabricating high-performance TFTs using low-cost, solution-processable methods to support the development of a unified, portable diagnostic platform. Earth-abundant materials - ZnO, and Zn–Sn–O (ZTO) - are investigated as n-type semiconductors, offering economic and environmental advantages. The benefits of stacking alternating ZnO and ZTO layers via spin-coating are systematically evaluated. Each system is characterised in terms of morphology and surface chemistry using X-ray techniques to link material properties with key transistor performance metrics. The study is further extended to assess the chemiresistive response of the devices to volatile organic compounds, including methanol and acetone.

To address the unipolar nature of metal oxide TFTs and their reliance on highly doped silicon substrates with thermally grown dielectrics, water-gated TFTs (WG-TFTs) are investigated as an alternative architecture. These devices integrate p-type organic semiconductors with n-type ZnO, mitigating limitations associated with all-organic channels. For the first time, this work explores how thermally-induced phase-separation in bulk heterojunction morphologies influences ambipolar charge transport - defined as the ability to support both electron and hole conduction under opposite gate biases within a single device. This ambipolarity enables inverter functionality and demonstrates strong potential for low-voltage operation and integration into clinical diagnostic technologies.