Amorphous Silicon Alloy Technology for Active Matrix Displays
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AMORPHOUS SILICON ALLOY TECHNOLOGY FOR ACTIVE MATRIX DISPLAYS Z. YANIV, V. CANNELLA, Y. BARON, A. LIEN, AND J. McGILL Ovonic Display Systems, Inc., 1896 Barrett Street, Troy, Michigan 48084* ABSTRACT Thin film semiconductor devices have been investigated over the past twenty years for application in large area flat panel displays. The development of thin film transistors and diodes based on amorphous silicon (a-Si) alloy materials has made the application of these devices, to display technologies, very attractive. More recently, manufacturing techniques to produce high quality large area films of amorphous silicon alloys have been demonstrated for photovoltaic applications. Most of the current research and development effort on active matrix liquid crystal displays (LCDs) has concentrated on a-Si alloy TFTs. The success of TFT based displays for large area flat panel displays has been limited so far, mainly due to the difficulty of obtaining a high quality gate dielectric by plasma deposition and due to the presence of crossing conductors on the same substrate, both increasing the probability of defects in the display. When a two terminal sandwich device is used, on the other hand, no gate dielectric is required, hence, a higher yield can be expected. Metal-insulator-metal and hydrogenated amorphous silicon alloy devices have been proposed for incorporation in LCDs. Performance requirements for a useful active matrix switching element and a comparison among the different a-Si alloy thin film devices used for this purpose will be reviewed. INTRODUCTION At the present time, flat panel displays without the bulkiness and power demands of CRT's are just entering commercial use. Further advances in flat display technology point toward large television screens as well as full size portable terminals. The two technologies emerging as the leaders are liquid crystals (using the active matrix technique) and electroluminescence. Recently advanced techniques for high multiplexing of twisted nematic LCD's have been applied to portable computers and pocket size televisions; however, the contrast ratio and viewing angle is not sufficient for such applications because they are limited by material (liquid crystal) properties. Large area matrix-addressable LCD's with a minimum of 250x250 picture elements operated in the fast update (TV) mode require the incorporation of an active electronic switch into each picture element. In an active matrix, the liquid crystal is driven in an effectively static mode, consequently excellent image quality can be obtained; however, the fabrication of such large area active matrices is similar to very large area microelectronic processing. As a result, special care must be taken in choosing the right active element and a simple panel design in order to obtain a high yield. The active elements are used for switching the information (charge) stored in each picture element. This switching operation has the following requirements: in the writing mode, the resistance of the active element must be low, and
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