Picture getting clearer on mobile phones

Mobile-phone shipments long ago passed PC shipments, but most mobile-phone displays have yet to approach the resolution PC monitors started delivering many years ago. The excitement at shows like CES centers around impossibly large flat-panel televisions, but display manufacturers gathered here for the DisplaySearch U.S. FPD Conference this week are starting to talk about the small screen.

The display is such a crucial part of the interface with a new system, no matter what the size, said Barry Young, an analyst at DisplaySearch. Right now, most mobile phones' displays feature QVG (320 pixels by 240 pixels) resolution or worse, which is a quarter of the original resolutions delivered by early PCs. But faster networks and processors are making it possible for mobile phones to do a lot more with video than they recently could, he said.

Unfortunately for content companies, watching even a short film on a mobile phone requires some compromises because of the limits of the display. Companies such as Samsung SDI, Sanyo, Epson and Sharp are working on improving those displays by borrowing some techniques from the large-display world and coming up with new ideas.

By 2010, 3.8 billion people are expected to own a mobile phone, which is about half the people projected to be living on Earth in 2010 by the U.S. Census Bureau, Young said.

Manufacturing anything in quantities that large is a demanding task, said Bo Polak, senior staff engineer at Motorola. The company finds it hard to properly evaluate emerging display technology because many small start-ups haven't tested whether their ideas will work for more than several million units, a small number for Motorola, he said.

But design flair is also a huge part of any mobile phone, Polak said. "It has to look good, and it has to enable compelling designs," he said.

Early advances in moving to VGA (Video Graphics Array, at 640 pixels by 480 pixels) resolution for mobile phones have involved shrinking common LCD design techniques down to 2-inch screens. Most mobile phones currently use what is called a passive-matrix LCD, but active-matrix LCDs used in notebook displays and flat-panel televisions are favored for their image quality and response time. Use of amorphous silicon and low-temperature polcrystalline silicon are two techniques for building active-matrix displays that show the most promise for small devices, according to DisplaySearch.

Polycrystalline silicon has traditionally been used to create large LCDs, but manufacturers mostly use amorphous silicon for advanced mobile screens because natural defects in that material don't have as much of an impact. They also can work with amorphous silicon at lower temperatures.

But polycrystalline silicon is more stable, and manufacturers have had encouraging results from a low-temperature polysilicon technique that now accounts for about a third of all small displays with active-matrix technology. Mobile phones are the main category, but digital cameras and portable DVD players are two other devices that come with active-matrix technology in small packages.

Low-temperature polysilicon is also being used to build displays for ultramobile PCs like the ones touted by Intel and Microsoft earlier this month.

Manufacturers like LCD technology because it's a known quantity. But those displays require a back light. A separate type, called OLED (organic light-emitting diodes), can produce high-quality images without a back light, which reduces weight and power consumption in a mobile phone.