PhD research, Imperial College London
Supervisor: Dr A. J. Campbell
Dates: 2010 - 2014
I developed a method to fabricate high-speed transistors on plastic film. This used a combination of lithographic approaches, as well as solution-processed gravure printing.
The processors that power our computers, smartphones and many other electronics are based on materials such as silicon - they're solid, brittle, crystalline. Plastics are known for their flexibility, relative low cost and insulating properties. However it turns out that not all plastics are insulating. Some are insulators, some are conductors, and some are semiconductors. These combine the electrical properties of inorganic semiconductors such as silicon with the material properties of plastics.
There are a huge range of carbon-based chemicals that exhibit semiconducting properties, including non-plastics too. These chemicals can be made to dissolve in solvents to create inks, before being coated onto plastic, paper and other materials. These can be printed using the same technology as found in an inkjet printer at home - or other methods such as a traditional newspaper printing press.
The ability to print these materials means we can create new types of technology - flexible circuit boards, smartphone screens and intelligent packaging. The printed packaging on your carton of milk could be smart enough to know when the milk is about to turn sour - reducing food waste.
In this work I designed the transistor architectures and process engineering required to fabricate organic field effect transistors (OFETs) and complementary circuits on plastic. In particular I focused on self-aligned architectures in order to minimise overlap capacitances between adjacent electrodes, resulting in faster operating speeds. In addition I fabricated sub-micron channel length devices using nanoimprint lithography, again yielding better performance. Finally gravure printed dielectrics and semiconductors were combined with these architectures to fabricate OFETs operating at > 5 MHz at < 30 V.
Cover image reproduced with permission. Higgins, S. G., Muir, B. V. O., Wade, J., Chen, J., Striedinger, B., Gold, H., Stadlober, B., Caironi, M., Kim, J.-S., Steinke, J. H. G. and Campbell, A. J. (2015), Organic Transistors: Self-Aligned Megahertz Organic Transistors Solution-Processed on Plastic (Adv. Electron. Mater. 5/2015). Advanced Electronic Materials, 1 doi: 10.1002/aelm.201570013