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As customers demand higher screen resolution with richer colors and refresh rates sufficient for eye-pleasing video, mobile display assembly designers must evolve their technology to minimize cost while delivering the required functionality.
One industry trend is undeniable: Display glass has become smarter. Low-temperature polycrystalline silicon (LTPS) thin-film transistor (TFT) displays are eclipsing standard amorphous-silicon TFT displays. The market share for active-matrix amorphous silicon TFT displays, while still dominant at 50 percent, actually declined 12 percent last year, according to research firm DisplaySearch. Most of the drop was due to increased shipments of LTPS TFT displays, which grew 30 percent and now comprise over a third of portable display shipments.
Samsung launched a display last September with the driver and timing controller functions integrated into it. Although this technology has been adopted for 7-inch and 5-inch displays, that is still too large for mobile-handset applications. Nevertheless, the technology indicates a trend toward integrating more electronics onto the display glass.
Samsung also recently announced the conversion of an amorphous-silicon TFT fabrication plant to LTPS. Its Cheonan plant is scheduled to begin processing LTPS substrates this month. NEC announced last November a 3.5-inch LTPS 960 x 540 (QHD) display with resolution of over 300 dpi.
As the industry evolves, we will see an increased occurrence of driver ICs that have been custom-designed for a specific display. Such is the case with Toshiba's 480 x 800 (WVGA) display used in the W52T mobile handset. The display provides full 18-bit color (262k colors), and the resolution of the 3-inch-wide screen is an impressive 266 dpi. It is believed that the display for this phone comes from Toshiba Matsushita Display Technology Co. (TMD).
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TMD's market share grew by more than 20 percent in 2006, strengthening the company's position vs. Samsung and market leader Sharp. TMD has completed a very optimized design with carefully balanced circuitry between the Toshiba T6K84 driver IC and the display itself.
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Significant design challenges have to be overcome to develop a single-chip display driver for a handset with this level of image quality. First, how can the designer incorporate 2,240 output drivers onto a single die? Currently, this is impractical. The tightest pad pitch we have seen on any display driver IC is about 50 mm on the staggered outputs on the Renesas BD663471R TFT source driver recently removed from Mitsubishi's 800iDS handset. Even at this pitch, 45 mm of IC perimeter would be required for a single driver chip to accommodate all gate and source-driver outputs unmultiplexed.
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In addition to the outputs, I/Os to the display controller also are required, resulting in a die area far in excess of any driver IC on the market. A large die area is not a good way to minimize system cost. Therefore, outputs to the display must now be multiplexed. This arrangement calls for some active switching elements on the display to control which rows and columns are accessed at any given moment.
The advantage of LTPS is that active components can be placed directly on the glass. While amorphous-silicon transistors exhibit poor electron mobility at less than 1 cm2/Vs, LTPS transistors have mobilities in excess of 400 cm2/Vs (traditional CMOS transistors are greater than 600 cm2/Vs).
