Michael Collis recently completed his SATROclub Extended Research Placement at the University of Surrey under supervision of Dr Radu Sporea. Michael was tasked with investigating next-generation electronic circuit applications made on large areas with unconventional technologies such as inkjet printing and resulting in flexible, low-cost and power efficient electronics.
Here's what Michael had to say about his time at the University...
This week I have been learning all about transistors, and preparing for the lab work that I will be doing during the next few weeks. First, I read part of “Electronics for Today and Tomorrow” by Tom Duncan, particularly the sections on semiconductors, diodes and transistors. I learnt the differences between n-type and p-type semiconductors (the former has electrons as charge carriers, the latter has positive “holes” as charge carriers), as well as about the junction that is created between the two when they are placed together. I then went on to learn about the junction transistor, which is the less common, but simpler type of transistor often used in high current applications, followed by the junction field effect transistor, and how they work. With this base understanding, I then learnt about Metal-Oxide-Semiconductor Field-Effect-transistors, otherwise known as the MOSFET, the most common of transistors today, used all the time in integrated circuits, which is very close to what my project is on. I went into more detail in these, also learning about its’ electronic characteristics and equations to model it, as well as how and where they are used. Finally, I applied this knowledge to the source-gated transistor, which my project is on. At this stage, I also learnt about possible production techniques, as well as the theory behind using an inkjet printer to create them. Finally, I made a general plan of the next three weeks with Dr Sporea, so that we can make the most out of the time available.
This week started with safety talks, first for the general labs, then for the clean room. I also obtained my amazing blue clean room suit. The first two days were getting used to the inkjet printer, which I printed a protective layer on copper covered plastic in certain designs (starting simple, but getter more complex on Tuesday). On Tuesday morning, I met Professor John M Shannon, the person who discovered and wrote the first paper on the Source Gated Transistor. On Wednesday, I etched my previously made designs in the copper in the clean room, before attempting to print silver electrodes on glass. I spent the whole day on Thursday in the clean room, preparing substrates, coating them in different substances and using masks, using the JLS Sputterer and other pieces of equipment, as well as making a semiconductor solution. During which, I made metal electrodes on glass, followed by a dielectric and them more metal, putting via holes in some of them, to test Via holes. On Friday I measured the current and resistance of the samples from Thursday, which gave out good results. Afterwards, I attempted to print a semiconductor ink, which failed due to clogging, before reading through numerous papers on Via hole printing so that we can try and improve our process.
This week I started by cutting around 30 glass substrates, for creating source gated transistors on. Before I could do this, I had to clean all the glass by first blowing each substrate with high pressure nitrogen gas, to get rid of any dust particles. Then, I placed them in beakers filled with acetone, and placed the beakers in a agitated water bath for five minutes. Next, I took them out of the water bath and rinsed them with acetone, before blow drying them with nitrogen gas. I then placed them in another beaker filled with isopropanol and then in the agitated water bath for 5 minutes. I rinsed the samples with isopropanol and blow dried with nitrogen gas. Finally, I placed them in a machine that fired high energy oxygen ions at the substrate, finally giving us a atomically clean substrate. Afterwards, I used the JLS sputter machine to place source and drain electrodes on the substrates, using different metals. Then I deposited two different semiconductors, insulator and finally a gate on the substrates. On Friday, I finally got to measure the electrical characteristics of the transistors; even though most had short circuits, the one that worked showed great electronic characteristics.