High-performance TFTs Thin-film transistors

The performance of STN (supertwisted nematic)-mode LCDs has been improved so far that STN displays can be used for many applications with low and high informational content [1], In particular, in price-sensitive applications they have proven to be competitive with the progressive TFT (Thin Film Transistor) technology. However, the usage of reasonable film-compensated STN displays has almost been restricted to environments with rather stable surrounding temperatures, which still is an obstacle for their widespread use, e.g. for car equipment or mobile applications. 

For high information-content displays, active-matrix (AM) pixel addressing provides improved display performance and reduced power consumption. In active matrix addressing each individual pixel is controlled by one or more thin-film transistors (TFTs). To date, most AM OLED displays have used polysilicon TFTs as the active elements, because they can provide sufficient current at low voltages and acceptable device dimensions, and they are capable of integrated drive electronics... 

The uniquely high mobility displayed by SWNT (146, 147) makes them attractive for applications in nanodevices, such as thin-film transistors (TFT), which could be produced by solution-processed random networks of SWNT. An optimum TFT would be composed entirely of semiconducting nanotubes, since their performance is limited by the presence of metallic tubes. It has been proposed that even below the percolation threshold of metallic tubes, electron hopping or tunneling may occur between neighboring metallic tubes (148). This electron channeling reduces the on/off ratio and therefore, the overall performance of the transistor. Removal of metallic nanotubes from the network has been achieved by electron breakdown (149, 150). 

Active-Matrix LCDs. Increase in FPD size along with demand for video response equivalent to the CRT made it necessary to avoid the high level of cross talk between adjacent pixels in passive displays. The nonlinear response of the liquid crystals was no longer suffieient and it became apparent that a switch was needed at each pixel. In principle, several switching technologies could be utilized since they all could be fabricated with films and photolithography. Metal-insulator-metal (MIM) devices, diodes, and transistors have all been tried. Thin-fihn transistors (TFTs) have performed the best and as a result have been adopted for most active-matrix applications. 

The ideal solution to the drive-address circuitry is to incorporate it directly into the thin-film structure on the glass. TFTs can be fabricated for the active matrix and the drive-address circuitry at the same time. This is already being done commercially for projection LCDs, where the small size and 1000 X 1000-line matrix restricts the number of connections that can be made to the substrate. A 950 to 1000°C process utilizing fused silica substrates (Table 7.1) is used to make the high-performance poly silicon transistors. Larger- area displays utilize the lower-cost flat glass shown in Table 7.6 where the strain point is far below that of fused silica. In this case, a laser recrystalUzation process is used to create high-mobihty silicon films for developmental displays. 

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