A creative combination of display technology and handset electronics is needed to implement the advanced cellular-telephone features that determine consumer choices.
by Chi Kwong Chow
THE PROBABILITY IS HIGH that most of us are carrying one or more displays most of the time. The miniaturization of electronics, advances in wireless digital communications, and the development of high-quality and relatively inexpensive small information displays have come together to create a burgeoning worldwide market for mobile telephones. As this enormous market continues to grow, it drives further advances in display technology.
Of all the applications for portable consumer-electronics products, mobile telephones command the largest market share. Global handset shipments for 2004 were estimated to be about 600 million units, which is more than ten times the size of the personal-data-assistant (PDA) or digital-still-camera (DSC) markets of about 40 million units. Projections of future sales predict that the PDA market is rapidly approaching the saturation point and will gradually merge with the mobile-telephone market.
In 2007, mobile-telephone sales will continue to grow, but individual PDA sales will drop to 14 million units per year (Table 1). Sales of camera phones with 1–2-Mpixel resolution are growing fast, indicating that this will become a dominant segment of the mobile-handset market in the coming years. The market consolidation of handheld devices will help fuel the growth of this market segment. Users are being encouraged to replace their handsets with new models that offer more multimedia functions. These new features add to the momentum that drives the market growth.
In addition to telephone conversations, mobile handsets can now perform a wide range of tasks, including text messaging, e-mail, still photos, video messaging, Web browsing, and games. The new messaging services help attract new subscribers to the convenience of mobile communications for both business and personal uses. Some devices even offer limited Word and spreadsheet features that let users exchange data with a desktop computer across a network.
The availability of these new services depends in large part on service providers. The greater use of GPRS and Gen 3 services, such as Wideband CDMA, CDMA2000, and Time Division Synchronous Code Division Multiple Access (TD-SCDMA), makes these multimedia services more widely available to consumers (Fig. 1).
Display Size and Resolution
The expanding range of mobile-telephone applications and features places increased demands on the display panel's performance and features. Handheld devices require displays in a wide range of sizes and capabilities. Lower-tier mobile telephones have displays in the 1.5–1.8-in. range, while mid-tier models tend to have panels in the 1.9–2.4-in. range.
PDAs and smart phones require even larger displays in the 3.0–3.5-in. range. Panels with 1.0–1.5-in. displays are popular for caller-ID sub-display applications in mobile telephones. These can provide caller information without requiring the user to turn on the larger primary display. This approach reduces system power demands, helping keep handsets smaller and lighter. The clamshell telephone – with its main and sub-displays – is the optimal handset design, making efficient use of system power, which is why this design has become more popular over the years.
(Source: ITIS, April 2004)
A display with a 2.4-in. diagonal is about the practical limit for handsets that consumers can operate with one hand. Simple messaging and update information can be handled adequately by panels with resolutions ranging from 96 x 64 to 128 x 160 pixels. Higher resolutions – such as 132 x 176, 176 x 220 (QCIF+), and 240 x 320 (QVGA) – offer reasonably good quality for viewing photos and playing games.
Displays with 3.5-in. diagonals are best for data applications, but are too large for single-handed operation. Panels of this size usually require a touch panel or voice control to manage the device and to input data for applications such as pocket-word-processor, spreadsheet, and Internet-browsing operations. Designs this large are definitely not convenient when making simple calls during shopping or when playing sports outdoors. In addition, larger displays generally require more power, so devices using such panels tend to be heavier to accommodate sufficient battery capacity, which may limit their appeal to consumers.
New panel technologies are being developed to improve resolution in order to make the use of handsets more enjoyable. Consumers will view finer pictures and graphics, but there are practical limits to how small the text characters can be and still be legible to the average user. As a result, the pixel format for 2.4-in. panels of low- and mid-tier handsets will not exceed QVGA in the next few years. And while manufacturers are developing 3.7-in. VGA-resolution panels for PDA and smart-phone devices, it is not clear whether the extra resolution is warranted.
FPD Technologies
After a decade of dominance, monochrome and gray-scale displays are being replaced by color panels in mobile telephones in order to handle the new multimedia functions. Black-and-white displays have dropped from a 90% market share in 2000 to less than 40% in 2004. Color technology is forecast to exceed a 70% market share by 2006. Numerous display technologies are being used to fill this demand.
Passive-matrix technologies include color supertwisted-nematic liquid-crystal displays (STN-LCDs), "Ultra Fine & Bright" (UFB) LCDs, "Ultra Fine & High Speed" (UFS) LCDs, and passive-matrix organic light-emitting-diode (PMOLED) devices. Passive-matrix LCDs are the most economical solution but they have some limitations, including slow response time, which is a handicap in video applications.
UFB, Samsung's designation for a passive-matrix display utilizing the super-birefringent effect, is the product of an extensive research effort intended to produce a better cellular-telephone display. It has the luminance, color gamut, and most of the contrast of a TFT-LCD, but only consumes as much power as an STN display, according to Samsung. It is currently used in a variety of Samsung cellular-telephone models and will probably be available to other handset makers.
UFS is Samsung's name for its field-sequential-color display, with technology licensed from Hunet. The technology is capable of video rates and high resolution.
The PMOLED features fast response time, wide viewing angles, and high contrast, and the emissive design eliminates the backlight. Unfortunately, it still has limitations in resolution and panel size. At present, these displays are only used as mobile-telephone sub-displays and in MP3 players.
The major active-matrix color-panel technologies are amorphous-silicon thin-film transistors (a-Si TFTs), low-temperature-polysilicon (LTPS) TFTs, and active-matrix OLEDs (AMOLEDs). Even though active-matrix devices consume more power than the passive-matrix devices, they provide superior multimedia performance for handheld devices, including fast pixel response time, high contrast, wide viewing angles, and high color depth and purity.
Some technology developers have also invested in continuous-grain silicon (CG-Silicon) devices, field-emission displays (FEDs), and plasma-display panels (PDPs). CG-Silicon technology enhances electron mobility up to 300 cm2/V-sec, more than that of TFT and LTPS technologies, and is best suited to mid-sized panels. FEDs and PDPs can only be employed in large-panel applications such as TVs and monitors because the high power consumption and large pixel pitch make them unsuitable for handheld devices.
The price gap between passive- and active-matrix technologies is the main reason that passive-matrix panels appear in the low-tier products, while the active-matrix designs are limited to higher-tier products (Table 2). The use of passive-matrix panels is likely to decline, however, as the price of LCDs drops 10–20% each year and the dollar difference between the technologies becomes smaller. As yield and stability improve, active-matrix LCDs will drop in price and their market share will continue to increase to more than 50% of the handheld-device market share.
Fig. 1: Mobile-telephone technology continues to expand in terms of features, functions, and transmission bandwidth.
IC Integration
In addition to a fast and colorful display, handheld devices require a powerful processor and a high-speed interface. The multimedia applications require high display refresh rates and massive data transfer of more than 2 Mbits/sec to support camera functions and streaming-video services such as videophones and TV broadcasting. Handset makers are determining which low-power high-bandwidth chip interconnects can support the next generation of camera phones. As many as eight different interconnects have been proposed at various levels, linking CMOS sensors, graphics chips, baseband processors, and displays. Several brand-name supplier groups have issued specifications for the development of their own standards. These include the Mobile Video Interface (MVI), the Mobile Pixel Link (MPL), the Mobile Display Digital Interface (MDDI), and the Mobile Industry Processor Interface (MIPI).
These interface standards were devised to reduce power consumption, electromagnetic interference (EMI), voltage swing, and current to lower the system noise level and the number of interconnection wires. Many designs rely on a serial interface with low-voltage-differential outputs. The interfaces will enhance the data-transfer reliability and mechanical com-pactness of multimedia mobile telephones, especially for those that use a clamshell design.
The existing LCD-driver controllers include the power generator, gate driver, source driver, and simple glue logic. To meet the demand for faster data-transfer rates, smaller IC-processor geometries are required. Most mixed-signal-process display drivers employ 0.35- and 0.25-μm technology. In 2005, 0.18-μm technology will be adopted in order to fulfill the data-transfer-rate requirements and encourage the integration of value-added features.
Value-Added Features
When making a buying decision, consumers consider the functions available in camera handsets. Such functions include voice recognition and synthesis, MP3 playback, and 3-D graphics animation. To combine these functions at low cost requires a high level of integration in the driver controller ICs.
For example, using current wafer technology, Solomon Systech has embedded a 2-D graphics acceleration engine into its driver controller ICs. This can support graphic animations such as drawing lines, rectangles, and circles, and copying block images by implementing graphics instructions in the IC controllers at the display module. This speeds up the entire application system by reducing the amount of data sent to the display, which in turn releases the processor's computing power for non-display-related features. Reduced demand on the processors permits a wider range of choice in the selection of baseband processors and decreases the demands for data-transfer rates across the interconnection.
Reducing the IC process to less than 0.18 μm will make it possible to integrate more features, such as the CMOS sensor timing controller, the 2-D/3-D graphics-accelerator engine, and JPEG/MPEG decoding. Such features can take full advantage of embedded SRAM as graphics memory.
These new features will require TFT- and LTPS-panel technology, and the high cost of panel tooling will create a barrier to the entry of manufacturers of low-tier handsets and panels. At least some of these display panels will have standard physical dimensions and resolution to serve second-tier manufacturers, making it easy to develop the display module as a platform solution or a stand-alone peripheral. By that time, the driver controller ICs will be a standard part of the handheld-display-module interface.
Conclusion
Handheld devices have transformed the way we work, play, and communicate. The mobile-telephone handset is emerging as the winner among competing devices, and, as a result, consumers expect it to perform more and more functions. Fortunately, technology advances in displays and the intelligence supporting those panels will make it possible for handset designers to deliver more features in smaller, more-power-efficient packages to respond to these increasing expectations. •