Electronic Design

Display Technologies Get Ready For The Challenges Of The Wireless Age

High-resolution color displays will deliver graphics and video as soon as high-bandwidth cellular networks come online.

Those who are curious about the future of display technology in communications may wish that they had access to that most ancient wireless display device: the crystal ball. This approach, however, has a number of drawbacks. While simple in construction, the spherical glass communicator with the hands-on user interface is tough to operate, typically requiring a trained specialist. Its use is further limited by a non-standard, poorly documented video interface that's only compatible with a poorly understood and unreliable communications "medium."

Even when reception is steady, image quality can be limited by low resolution and distortion caused by curvature of the glass. Image size, brightness, and viewing angle also may be restricted, because users are forced to hover over the display. Advocates might point out that the device's very low power consumption makes it a candidate for some portable applications. Unfortunately, its fragility, weight, size, and odd form factor automatically disqualify it for service in most of today's handheld communications products.

Luckily, there are alternatives to the crystal ball. Current market trends give us some indications of the display performance levels that will be required a few years from now. As a starting point, consider cellular phones. According to David Mentley of Stanford Resources, San Jose, Calif., the cell-phone industry can be expected to generate $2 billion in revenue by 2003. Given the size of this market, its products will clearly drive much of the development of portable-display technology.

Existing requirements for these products' displays will either remain or grow more stringent. Designers will aim for smaller size, lighter weight, lower power, less cost, better readability in variable ambient lighting, and greater durability. Take power, for example. An existing paradigm for mobile phones is that they must provide eight hours of operating time and 24 hours of standby time on a single battery charge. But manufacturers are pushing to raise these numbers through the development of better batteries and more power-efficient designs.

On the battery side, improvements in lithium-based cell chemistries, such as Li-ion, Li-polymer, and Li-S, will produce higher energy densities.While some of this additional energy will probably go to increasing operating time, it also will give designers the option of switching from monochrome to the more power-hungry color displays. Of course, display vendors will be working to reduce this power penalty by developing more efficient LCD and alternative technologies.

The demand level for displays that can deliver high information content is going to depend largely on the pace of development in wireless networks. Today, those networks typically provide data communications at 14.4 kbits/s, which limits them to tasks like downloading e-mail and text-only web sites. Such applications can be handled by the low-resolution LCDs currently found in portable cell phones and wireless-enabled PDAs. But as wireless service providers upgrade their networks to third-generation standards, data rates will rise into the range of hundreds of kbits/s and beyond, allowing transmission of color graphics and video.

For wireless phones and other handheld information products, the need for color graphics and video will grow as more of them connect to the Internet. The Gartner Group, Stamford, Conn., states that by 2004, 70% of new cell phones and 40% of new PDAs will use wireless technology for direct access to web content and enterprise networks. Increases in the bandwidth available for such data connections will move us beyond text-based web access, to the live-action color graphics we've grown accustomed to on our desktop and notebook PCs.

Indeed, expectations for portable devices have been raised substantially by the quality of images produced by the active-matrix LCDs (AMLCDs) found in our notebooks. Factor in the emergence of communications products with built-in digital cameras for transmitting still or moving images, and those low-resolution monochrome displays run out of steam.

A transition to 800- by 600-pixel SVGA looms ahead, as this is the standard specified by Windows that also accommodates both NTSC and PAL video. To save power, though, some applications will likely opt for lower resolution, such as quarter VGA.

Jumping up to SVGA will require a move away from the direct-view LCDs that are now prevalent. In current cell-phone designs, displays are generally monochrome passive-matrix LCDs with 1- to 2-in. maximum diagonal measurements and resolutions in the neighborhood of 60 by 90 or 120 by 90 pixels. For handheld organizers or PDAs, which are making inroads as communications devices for e-mail, the displays tend to run a bit larger. They go up to about 3.9 in. with quarter-VGA (320 by 240) resolution. In certain cases, vendors have begun opting for the more expensive, thin-film-transistor-based AMLCDs to obtain color. But in doing so, they will sacrifice some battery life.

Even though these direct-view LCDs are ill-equipped to handle heavy graphics and video, they probably won't disappear anytime soon. Improvements in their design will produce thinner and lighter displays, with lower power and greater brightness and contrast. Sharp Microelectronics has gone into production with a liquid-crystal-on-plastic display. It is said to produce thinner, lighter, and more reliable displays than those fabricated with the usual liquid-crystal-on-glass. In the future, such an approach may foster the creation of cell phones that are thin enough to withstand flexing.

The trend toward thin-film-transistor (TFT) AMLCDs will continue as demand for color grows. Supporting this trend will be the ongoing development of low-temperature polysilicon TFTs as an alternative to amorphous-silicon TFTs. Building the transistor array in polysilicon allows the integration of peripheral circuits, such as drivers, on the display's substrate. Ongoing efforts to reduce power also will help, as manufacturers push to improve on liquid-crystal performance and develop more efficient drivers. They will also work to reduce losses in the transistor array and incorporate more efficient backlighting.

A more radical change will be the growth of microdisplays. When viewed under magnification, these 1.5-in. or smaller devices can produce an SVGA or higher-resolution image. That's while consuming a fraction of the power of an equivalent but physically larger TFT display. In portable applications, the display is magnified by a viewfinder that's either housed in a headset or handheld unit—perhaps embedded in the communications device. A virtual image is created that appears recessed in the viewfinder.

Say you're watching a 0.5-in. microdisplay in the viewfinder. It may create the effect of watching a 15-in. display at a distance of 12 in. from the eye. Optics will play a key role in determining the success of these devices, because they'll affect image quality and the comfort of the user. The microdisplay will probably be an add-on or plug-in accessory for cell phones and other mobile products, which will still offer direct-view screens for dialing, messaging, and other tasks.

LCOS For Microdisplays
At present, different technologies are vying for the emerging microdisplay market. Liquid-crystal-on-silicon (LCOS) appears to have the edge, with actual products currently available (Fig. 1). Two methods are used to fabricate LCOS: polysilicon, which is an extension of traditional AMLCD technology; and single-crystal silicon, also known as silicon-on-insulator (SOI). The former approach is popular with Japanese display companies, while single-crystal silicon is favored by U.S. manufacturers.

Transistors made in single-crystal LCOS can be made smaller, allowing denser packing of pixels. This, in turn, leads to smaller displays. Single-crystal silicon also produces transistors that are fast enough for field-sequential operation, a method whereby a color image is produced by strobing one set of pixels with red, green, and blue light. Polysilicon is too slow for this operation, so its displays resort to the use of subpixels covered with red, green, and blue filters. This ultimately lowers spatial resolution.

Graphics output usually is in spatial RGB format, so an ASIC is required to convert this data into the field-sequential format. In time, OEMs will most likely incorporate this function into their graphics controllers, which will free up some board space and save on cost and power.

On the other hand, display vendors might choose to integrate the field-sequential conversion function into their silicon. Part of the attraction of single-crystal LCOS is that it provides a path for integration. The silicon in LCOS could be standard CMOS in a 0.5- or 0.35-µm process. They might be putting drivers on the display silicon now, but expect other functions to be incorporated into the display in the near future. These functions could include core processors, gamma processors, and color tables.

LCOS also holds promise because as silicon design rules shrink, the display can be made smaller. Voltages and power also can be reduced. But concerns remain about production yields, which must be improved to make LCOS feasible. The timing of this progress is critical, as it faces competition from technologies like organic light-emitting-diode (OLED) displays.

As their name implies, OLEDs are emissive-style displays, so they don't require backlighting. In one design approach, the basic OLED cell contains a series of carbon-based (organic) layers stacked in between a transparent anode and a metallic cathode. Within the stack are a hole-injection layer, a hole-transport layer, an emissive layer, and an electron-transport layer.

Applying a few volts to the OLED cell causes positive and negative charges to recombine in the emissive layer, which then generates light. By doping the emissive layer with fluorescent molecules, the designer allows the cells to produce color output. These cells can form into both passive-matrix and active-matrix versions of OLED displays. The latter are produced using polysilicon TFTs.

OLEDs have been gaining attention because they offer several advantages over conventional LCDs. They sport much wider viewing angles, going as high as 160°. The displays also feature greater brightness and contrast, more uniform light output, lower power consumption, and thinner packaging.

OLEDs Show Potential
The first applications of OLEDs were passive-matrix displays for car audio equipment. More recently, Eastman Kodak Co. and Sanyo Electric Co. introduced a full-color, active-matrix model that hints toward the OLEDs' greater potential. This 2.5-in. display with quarter-VGA resolution is a mere 1.8-mm thick, compared to 6 or 7 mm for an equivalent LCD (Fig. 2). While a backlit AMLCD would devour 800 mW of power, the OLED's power consumption is approximately 300 mW.

The plans at Kodak call for the development of bigger displays with higher resolution, including a 5.5-in. version with VGA resolution. Like some of its LCD counterparts, the 2.5-in. OLED display was built using low-temperature polysilicon that permitted the integration of row and column drivers. Down the road, control and DSP functions may be incorporated.

Kodak's display should be in production somewhere around 2001. This will provide some interesting competition for the more traditional direct-view AMLCDs.

The structure of the OLED actually makes microdisplays a possibility, as well. Because the OLED is an emissive device, the display aperture factor isn't significant like it is in LCDs, which modulate light from an external source by passing it through an aperture. As a result, pixel count, resolution, and size can be scaled down to produce microdisplays. Treading down this path is FED Corp., which has a 0.78-in. diagonal display in development that offers 256-level gray-scale and SXGA resolution. The company's roadmap calls for even further development of XGA and SVGA versions.

According to Gary Jones of FED Corp., an OLED microdisplay will have advantages over LCOS for applications like cell phones that would employ a microviewer or headset. With an OLED display, the microviewer wouldn't need to be held as precisely to see the complete image. Plus, its greater viewing angle would allow for smaller and cheaper optics. OLED isn't strobed like LCOS, so its display wouldn't suffer any color separation when vibrations are present, such as when the viewer is riding in a car or train. Lower power is another advantage for OLED over LCOS.

For now, though, LCOS microdisplays appear to be ahead in the race to bring high information content to the portable world. It remains to be seen how LCOS and OLED technology will fare against each other in the microdisplay arena and versus the steadily improving direct-view AMLCDs. The final outcome will put a new face on cell phones, PDAs, and other handheld communications devices. It also will change the ways that we interact with one another via the wireless world.

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