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Posted on July 23, 2009 by Dr Peter Harrop

Nano ePrint developing printed nano-transistors in zinc oxide

Nano ePrint, formerly Plastic ePrint, the 2006 spin out from Professor Aimin Song's group at Manchester University in the UK, continues the development of its remarkable single layer transistor printing system.
While the hundreds of others developing printed transistors concentrate on field effect transistor geometry of decades ago, with attendant challenges of printed feature size, frequency, voltage, interconnects and finding high permittivity, high dielectric strength gate dielectrics to deposit, Nano ePrint neatly by passes all that with a single layer planar nano-transistor (PNT) structure that has no gate dielectric. Planar nano-diodes (PNDs) can also be constructed via a similar approach.
Using this architecture, Nano ePrint can fabricate electronic devices in a single layer of thin-film semiconductor, deposited on a low cost flexible substrate, using a single-step patterning approach. The patterning is performed using nano-embossing, analogous to the earliest forms of printing (wood block, letterpress, etc) but on a much finer scale. This printing process is compatible with roll-to-roll, and has been in use commercially for around 20 years for the production of optical microstructures such as the security holograms that are embedded in modern banknotes and passports.
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Because PNTs and PNDs are fabricated within the semiconductor layer itself, they do not require dielectric materials, or conductive contacts for source, drain and gate. Eliminating this conventional multi-layer material stack also eliminates the registration issues of aligning successive material or process steps while dealing with a flexible substrate. This allows much smaller feature sizes, resulting in both dramatically reduced circuit footprint and significantly increased performance.
Interconnect is also improved using Nano ePrint's approach. For relatively simple circuits or functional units - such as logic gates (AND, OR, NOT, etc) and basic analogue blocks (rectifier, amplifier, modulator, etc) - the interconnect between adjacent PNTs and PNDs can be achieved directly within the semiconductor layer. For more complex circuits, a separate conductive interconnect layer may still be required, but the routing problem is substantially simplified.
CEO Scott White tells us that the company first demonstrated organic versions but the focus is now zinc oxide based semiconductors with their superior properties. InGaAs versions working at terahertz frequencies have also been demonstrated but the main market potential for the company - now consisting of about ten people - lies in zinc oxide devices working at up to GHz frequencies, he reports.
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Printed zinc oxide and titanium oxide can act as semiconductors or dielectrics depending on morphology and doping. For example, InGaZnO and similar formulations appear in the printed transistors and semiconductors being developed by Tokyo Institute of Technology, Toppan Printing, Kodak, Panasonic, Sanyo (now owned by Panasonic), Canon, NEC, Samsung, ITRI Taiwan, Merck Chemical, Technical University of Darmstadt, Technical University of Lisbon, University of California San Diego, Cambridge University Centre for Applied Photonics and Electronics and 3T Technologies.
The Hewlett Packard versions have been licensed to electronics company Inpria and photovoltaics company Xtreme Energetics. Like them, Nano ePrint sees more than an advantage of an order of magnitude improvement in mobility and therefore frequency expanding the addressable market over organics. ZnO based semiconductors are transparent which also greatly widens the market opportunities, not least in brand enhancement of consumer goods, where circuits can be placed over existing artwork rather than stealing real estate.
Like the silicon chip, success with printed transistor circuits depends on modularity and configurability. Basic hardware platforms configurable to the needs of all industries from military to consumer packaged goods will be developed by the more savvy putative suppliers - the low cost label with a scrolling message and one that records and plays back are examples.
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