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Posted: Jun 12th, 2007
Transparent and flexible electronics with nanowire transistors
(Nanowerk Spotlight) Thin-film transistors (TFTs) and associated circuits are of great interest for applications including displays, large-area electronics and printed electronics (e.g. radio-frequency identification tags - RFID). Well-established TFT technologies such as amorphous silicon and poly-silicon are well-suited for many current applications - almost all mobile phone color screens use them - but face challenges in extensions to flexible and transparent applications. In addition, these TFTs have modest carrier mobilities, a measure of the velocity of electrons within the material at a given electric field. The modest mobility corresponds to a modest operating speed for this class of TFTs. Organic TFTs are generally better suited for flexible applications, and can be made transparent. However, mobilities in organic TFTs are generally quite low, restricting the speed of operation and requiring relatively large device sizes. Researchers at Purdue University, Northwestern University, and the University of Southern California now have reported nanowire TFTs that have significantly higher mobilities than other TFT technologies and therefore offer the potential to operate at much higher speeds. Alternatively, they can be fabricated using much smaller device sizes, which allows higher levels of integration within a given chip area. They also provide compatibility with a variety of substrates, as well as the potential for room-temperature processing, which would allow integration of the devices with a number of other technologies (e.g. for displays).
"We have demonstrated fully-transparent thin-film transistors (TFTs) on both glass and flexible plastic substrates" Dr. David B. Janes tells Nanowerk. "The TFTs utilize wide-bandgap semiconductor nanowires as the active channels, and transparent conducting oxides for the gate, source and drain electrodes. The transistors exhibit good performance characteristics, including relatively high on currents (up to 10 microamps per nanowire) and high on/off current ratios (required for digital applications in order to achieve low power operation)."
Janes is a professor at the School of Electrical and Computer Engineering, Birck Nanotechnology Center, at Purdue University. Together with collaborators from the Institute for Nanoelectronics and Computing at Northwestern University and the Department of Electrical Engineering at the University of Southern California, he and colleagues from the Birck Nanotechnology Center fabricated fully transparent and mechanically flexible nanowire transistors (NWTs) on plastic substrates ("Fabrication of fully transparent nanowire transistors for transparent and flexible electronics").
Image of NWTs on a plastic substrate, showing the optical clarity and mechanical flexibility. Arrows point to the transistor array regions (Image: Dr. Janes)
The combination of excellent optical transparency and mechanical flexibility of In2O3 and ZnO nanowires, as well as excellent device performance metrics (they have an optical transmission of ∼81%), make these NWTs an attractive technology for realizing
transparent and flexible circuits. Fully transparent NWTs will not only increase aperture ratio efficiency in active matrix arrays, but will also enable low-power consumption as well as transparency for future display technologies.
"Prior reports on semiconductor nanowire transistors have typically employed non-transparent metal electrodes, making the overall structure relatively opaque" explains Dr. Tobin J. Marks, Vladimir N. Ipatieff Professor of Chemistry and Professor of Materials Science and Engineering at Northwestern University, and one of Janes' co-authors. "Our study demonstrates that nanowire electronics can be fully transparent, as well as flexible, while still maintaining high performance levels. This opens the door to entire new technologies for high-performance transparent flexible displays."
The reported nanowire TFTs would be well suited for the drive circuitry in active-matrix displays, and would be compatible with pixel technologies such as organic light-emitting diodes (OLEDs). The devices are fabricated using low-temperature processing techniques, which allows integration onto plastics and various other substrates in order to achieve flexibility and ease of packaging.
In particular, there are three broad application areas for this work:
1) Transparent displays: Transparent displays are of interest for applications such as heads-up displays on windshields and informational displays on eyeglasses.
2) Flexible displays: Emerging applications such as "e-paper" require flexible electronics to be integrated within the pixel array. The demonstration of reliable TFTs on flexible substrates represents an important step toward the required circuitry. While TFTs suitable for static images have been demonstrated using organic thin-film transistors, the nanowire transistors demonstrated in this work could operate at much higher speeds, and would allow full-motion video.
3) Transparent/flexible electronics: Applications such as electronic bar codes, RFID tags, and smart credit cards would be advanced by the availability of relatively high performance electronics that could be integrated on a variety of substrates. Flexible circuitry would allow integration on curved and non-rigid surfaces. Transparency would allow integration into multi-layer packaging, in a fashion such that product information could be seen beneath the electronics.
"The demonstration of individual TFTs with the characteristics we reported represents a first step toward the circuitry required for many novel display and flexible electronics applications" says Janes. "We expect future studies to include demonstrations of analog and digital circuits based on the nanowire TFTs. This will require integration of multiple TFTs into an interconnected circuit, as well as development of appropriate interconnect approaches to provide the specific circuit topology."
Further reading on transparent electronics can be found in some of our previous Spotlights: