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.
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