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Printed Electronics World
Posted on August 26, 2009 by  & 

Printing silicon in South Africa - Part two

Printed silicon transistors are now receiving increasing attention because they can provide good transistor characteristics at low cost on flexible substrates, including high frequency performance. It used to be believed that, because printing organic transistors is relatively easy and they do not call for high temperature annealing, they would be the lowest cost and address the largest market potential. This may still turn out to be the case. Indeed, Plastic Logic is near to commercialising printed organic transistors as back plane drivers in their e-readers and PolyIC has demonstrated basic RFID and indicators using this technology.

Why print organic and zinc oxide transistors?

Organic transistors can be ambipolar and emit light and that may provide outlets for the technology. However, progress in increasing the charge carrier mobility and thus the operating frequency has been inferior to progress with alternatives and organic transistors are not yet fully transparent as required by those developing invisible electronics.
Temperature range and stability are not the best either. Zinc oxide semiconductors permit printed transistors to be fully transparent and there is progress in printing them, though some proponents have to do a high temperature anneal on high temperature substrates and sometimes indium, a material subject to price hikes, is used both in the semiconductor and the transparent electrodes.
RF sputtering of zinc oxide transistors permits low temperature plastic film substrates but it is less easy to make low cost/ high volume than printing processes. N type is troublesome with organic semiconductors and p type is a problem with zinc oxide transistors, though options do now exist for both. With both organics and zinc oxide there are challenges of large feature size, making such things as the 72,000 or more transistors in a small ISO 18000 RFID label something of a distant dream.

Where nanosilicon fits in

That brings us to the third option, which is printing doped nanosilicon particles where there is no problem with n or p type, mobilities and therefore frequencies of operation and addressable market can be best of all. Feature size can be very small. However, high temperature anneal and flexible stainless steel substrates are currently needed for the best performance and the market could also use something inherently lower cost and compatible with other components currently printed on low cost polyester substrates. With printed electronics, component integration is extremely important in optimally addressing market needs.
One organisation making rapid progress in the low temperature printing of flexible nanosilicon transistors is the University of Cape Town in South Africa.
Its value proposition is:
"Printed silicon is a fully printed technology which fulfils all the requirements of printed electronics, which are only partly met by competing technologies such as organic and hybrid electronics. These are:
  • Direct additive patterning
  • Room temperature processing
  • No post-processing
  • Design flexibility and scalability
  • Low capital and processing costs
  • Low cost abundant source materials
  • Environmentally friendly materials and processes.
Also, printed silicon is based on silicon - the most well understood and widely used semiconductor material in existence."
Reported achievements include traditional screen-printing of nanosilicon field effect transistors, using purely additive patterning technologies at room temperature. There are no additional postprocessing steps.
Transistors have been produced on paper substrates. (IDTechEx believes that there is much larger potential for paper electronics than is reflected in current research programs mainly centered in Finland, Sweden, Portugal and Japan).
So far, Professor David Britton and Professor Margit Härting in Cape Town University report performance characteristics comparable to amorphous silicon thin film transistors and the better organic transistors. Insulated gate field effect transistors employing n type silicon in the semiconductor layer operate in accumulation mode with effective carrier mobilities in the range 0.3 to 0.7 cm2 /V/s. For comparison, the better nanosilicon or zinc oxide transistors using high temperature annealing achieve tens of cm2 /V/s but the heating greatly restricts co-deposition of multi-component devices at lowest cost.
For its low temperature process and inks, 6 patent applications have been filed by the University of Cape Town through the PCT system in all major territories, including USA, Europe and the Far East. Two patents have already been granted in South Africa.
Envisaged markets are described as follows:
"Printed silicon technology is ideally positioned to provide solutions for electronic applications where combinations of attributes like light weight and portability, robustness and mechanical flexibility, and the choice of materials which the devices are deposited on, cannot be met by conventional technologies.
The applications are:
  • Active electronics: pixel switches in displays for e.g. cellular telephones and eReaders; memories used in applications like RFID tags and disposable ticketing; and signal processing.
  • Solar cells: charging units for portable electronics and telecoms; low power lighting; and light duty household use in areas without grid access.
  • Passive electronics: sensors for medical diagnostics, intelligent packaging and environmental monitoring.
Using environmentally friendly/renewable materials, as well as benign production processes, combined with cost effectiveness presents opportunities for printed silicon to be the universal platform for disposable electronics, opening avenues to a multitude of new products."
Industrial and commercial partners are being sought to help position the different aspects of the platform technology, and to leverage the maximum impact through direct investment, joint ventures or other collaboration models.
IDTechEx believes that there is a place in the emerging market for several transistor technologies. After all, some prospective users such as the consumer packaged good companies demand simple performance, only two-year life and very low cost and good environmental credentials. Military and industrial applications will usually emphasise 20 year life and superlative electronic, reliability and temperature performance, often with very small feature size.
Source top image: University of Cape Town

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Posted on: August 26, 2009

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