New realities for display drivers

The market for display driver ICs is attempt- ing to adjust to two con- current developments. One centers on commoditization; the second involves the rivalry between traditional, amorphous silicon (a-Si) and low-temperature polycrystalline silicon (LTPS) active-matrix displays.

There is no denying that display driver ICs have become a commodity. If you tally the display drivers in your laptop, desktop monitor, mobile phone, flat-panel television, PDA, car navigation system and portable gaming unit, you are likely to be the personal owner of more display driver ICs than DRAM chips. And when the volumes shipped of a particular device approach or even surpass DRAM volumes, it means the device in question has officially achieved commodity status.

When a device becomes a commodity, the main area of focus for product development becomes cost, as handset and monitor manufacturers demand lower component prices and as chip makers try to squeeze as much margin as possible from each chip design.

For example, two recent display drivers targeting high-end mobile devices managed to reduce die area significantly. The NEC µPD161704 five-metal-layer, 0.25-µm device measures 24.5 x 2.2 mm, and the Renesas Technology 61505 four-metal-layer, 0.18-µm device measures 21.5 x 1.2 mm. Of course, newer lithography nodes are more expensive, but achieving a die area reduction of greater than 50 percent is commercially compelling.

The most recent iSuppli data (Sept. 18) shows the rapid adoption of LTPS display technology at the expense of all the other display types. For the first time, the market share of a-Si displays has shrunk. Of course, display driver IC companies have addressed this market dynamic by launching driver chips with the appropriate drive signal characteristics for LTPS thin-film-transistor (TFT) active-matrix displays.

The more daunting reality for the driver IC companies, however, is the move toward integrating more of the display electronics directly onto the display glass, enabling significant cost improvement through component reduction. And the first candidate for this technology is the display driver IC. In September, Samsung launched a 7-inch liquid-crystal display with all drivers and timing controller on the glass substrate.

Traditional a-Si TFTs are the heart of the raster structure in active-matrix LCD displays. These transistors exhibit very poor performance, with electron mobility less than 1cm²/Vs, compared with >600 cm²/Vs for traditional CMOS. Poor electron mobility prevents the integration of the display drivers onto the glass. By annealing the deposited silicon into a more crystalline structure, electron mobilities approaching that of traditional CMOS are possible. Oven annealing can increase mobility to about 150 cm²/Vs; laser annealing can yield >400 cm²/Vs.

Two things become possible at these levels of performance. First, the active transistors in the pixel structure can be much smaller than their a-Si cousins, resulting in a better aperture ratio (i.e., ratio of the transistor size to the pixel size). Since the LCD backlight doesn't pass through the transistor itself, smaller LTPS TFT transistors result in a brighter display (or one that requires a less power-hungry backlight). Further, the devices' faster speed enables more-pleasing motion video. Finally, high-mobility TFT transistors on the LCD glass substrate enable the incorporation of display drive circuitry, eliminating the drive ICs entirely.