Thin Film Transistor Technologies

Flat-panel displays (FPDs) are becoming increasingly commonplace in today's commercial electronic devices. FPDs are finding widespread use in many new products, such as cellular phones, personal digital assistants (PDAs), camcorders, and laptop personal computers (PCs). This generation of handheld electronics places stringent demands on their displays. FPDs in these devices are expected to be lightweight, portable, rugged, low-power and high-resolution. Displays having all these attributes will enable a wide variety of commercial applications in the future.

 Active-matrix liquid-crystal displays (AMLCDs) are the leading flat-panel display technology. These displays are ubiquitous in laptops, often dubbed "active-matrix TFT," (an abbreviation for "active-matrix thin-film transistor"). What exactly does this name mean?

 A display is composed of a grid (or matrix) of picture elements ("pixels"). Thousands or millions of these pixels together create an image on the display. Thin-film transistors (TFTs) act as switches to individually turn each pixel "on" (light) or "off" (dark). The TFTs are the active elements, arranged in a matrix, on the display. Thus, the name "active-matrix TFT."

 Most commercially available AMLCDs use glass as the starting material in the display fabrication process. Glass has excellent optical clarity and is compatible with chemicals used in standard semiconductor processing. However, glass has the undesirable characteristic of being extremely fragile. As a result, displays must be handled carefully to avoid breakage. However, if plastic is employed as the starting material for display fabrication, we can achieve a display that is not only lightweight and rugged but also flexible. The realization of such a technology will have a significant impact on the display industry. It is, however, not a trivial task to fabricate displays on plastic. Many significant challenges arise when plastic substrates are used in place of glass. Research in the TFT group is aimed at addressing and overcoming those challenges.

The development of a thin-film transistor (TFT) technology for use with plastic substrates is still in its infancy. There is significant room for improvement in ultra-low temperature fabricated poly-Si TFTs. High mobilities, low leakage currents and threshold voltages are desirable for high-performance active-matrix LCD applications, particularly for the integration of driver circuitry, but low processing temperatures (<150ºC) must be maintained for compatibility with low-cost plastic substrate materials. In general, superior poly-Si TFT performance is achieved with higher-temperature fabrication processes, because the quality of the critical gate-dielectric interface is highly sensitive to process temperature.

 An ultra-low-temperature (100ºC) fabrication process, which would be compatible with flexible plastic substrates, is being developed by the TFT technology group at UCB. The goal is to achieve polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) with current-driving capability far exceeding that of conventional amorphous-silicon TFTs typically employed in high-performance active-matrix liquid-crystal displays today. Techniques for formation of the poly-Si and gate-dielectric materials are being investigated in order to determine the optimum processes for high-performance TFTs. Various device and process architectures for attaining low leakage current are being studied. The degradation of TFT performance under high-voltage bias stressing will also be characterized. To enable device modeling and circuit design, a physically based model for ultra-low-temperature-fabricated TFTs will be developed.

 Contributed by Yeh-Jiun Tung
Click here for a paper on Silicon Transistor Fabrication on Plastic Substrates.

This work is supported by a grant (ECS-9733247) from the National Science Foundation, under the Career Advancement Program.

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Definitions of Terms

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[Carey97] P. G. Carey, P. M. Smith, P. Wickboldt, M. O. Thompson and T. W. Sigmon, Conference Record of the 1997 International Display Research Conference (Toronto, Canada), pp. M36-M39, 1997

 [Young96] N.D. Young, R.M. Bunn, R.W. Wilks, D.J. McCullough, G. Harkin, S.C. Deane, M.J. Edwards, and A.D. Pearson, EURO Display '96, pp. 555-558, 1996

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