Abstract
In order to deliver billions of cost-effective sensors for Internet of Things applications, large area electronics (LAE) would need to overcome challenges in high-throughput manufacturing methods, while providing power-efficient operation. Processes, such as roll-to-roll and/or inkjet printing, hold the most promise for future disposable and wearable technologies, however progress is limited despite growing demand. The reason is the fundamental electronic device at the heart of their circuits, the thin-film transistor (TFT). TFTs share the same device physics as silicon field-effect transistors found in chip technology, as well as their limitations, notably uniformity of operation. Ordinarily, device non-idealities are compensated through cascode circuits or gain stages, which increases circuit complexity. But these strategies reduce yield, as circuit failure increases with component count. Even though there have been many breakthroughs in material systems, the limitations have persisted. Thus, the ability to produce high yield in high-throughput methods requires TFTs with more robust and uniform operation that can tolerate imprecise processes.
An alternative, the source-gated transistor (SGT) is a type of TFT that uses energy barriers at the source contact to control charge injection. As a contact-controlled architecture, the SGT provides uniform and robust operation with extremely high gain and power-efficient operation.
In this thesis, SGT operation is further explored in light of off-state behaviour, which produces lower leakage current. The first source-gated transistor (SGT) circuits are also included and provide exemplary performance without support circuitry. Two-transistor (2T) circuits with polysilicon SGTs demonstrate: 49 dB gain in common-source amplifiers, a record for any polysilicon TFT-based amplifier; and current mirrors that have a tuneable temperature dependence by design of the source region, where positive, neutral or negative dependence of output current can be obtained.
The SGT’s ability to provide superior high-gain and power-efficient performance in a compact footprint is only superseded by that of the newly invented multimodal transistor (MMT), which shares its benefits. The MMT is an evolution of the contact-controlled concept, where charge injection is separately controlled from channel conduction. As the channel in the MMT is not responsible for charge injection, it provides: faster digital switching (up to two orders-of-magnitude faster than SGTs and one order faster than regular TFTs); an alternative means of charge transport control to mitigate hot-carrier effects in high mobility materials, when doping strategies are unavailable; a unique sample-and-hold or enable line function, ensuring signal propagation when activated. Together with the inherent ability for the MMT to produce a directly proportional dependence of output current on input voltage, these functional benefits allow for extremely compact digital-to-analog conversion and multiplication. Examples of the MMT’s ability to reduce circuit complexity would allow for circuit designers to explore new avenues into developing future LAE applications.