Flat-panel displays

Nov. 15, 2002
Liquid-crystal displays are helping manufacturers meet consumer demand for smaller, more portable products.

Liquid crystals: Liquid-crystal displays are helping manufacturers meet consumer demand for smaller, more portable products. LCDs draw less current and run on lower voltage than comparable displays of light-emitting diodes. The also are more difficult to drive because they rely on complex chemical properties.

LCDs consist of liquid crystal sandwiched between polarizing glass plates. Liquid-crystal molecules are elongated in shape and align themselves to external electric fields. Molecular orientation determines how the material reflects light. Display segments are made visible or translucent by electrically aligning localized molecules to the polarizing plates. Voltage is applied across electrodes on the front and back planes of the assembly.

LCDs are available as either graphic, pixel-addressable devices or as alphanumeric character displays. Alphanumeric devices are frequently used in equipment such as handheld terminals and various kinds of industrial control panels. Typical configurations are eight lines by 40 characters. Most such panels are packaged with optional fluorescent or electroluminescent back lighting. Graphic displays are available that range from full-screen laptop computer size to 640 X 200-pixel half-screen sizes. As with CRTs, LCDs can be packaged with touchscreens to provide integrated operator panels.

Full-color LCDs are employed in applications where they can help operators distinguish intensity levels. Typical formats include units having 8.7 and 10.7-in. diagonals that provide a VGA-compatible display.

One way microcontrollers drive LCDs is through a multiplexed interface. This method is suited for displays comprising a large number of segments in matrix arrangement. However, it requires a complex circuit that generates several intermediate voltages to safely control display segments. Commercial LCD drivers for medium to large displays commonly provide this function.

If the display has only a few characters, an easier way to control it is with a static drive. This method involves individual connections to each display segment, and it eliminates the need for an interface IC. Static drive is similar to the control method used for seven-segment cathode displays. It employs a common backplane driver and individual segment drivers for the electrodes on the front plane. The interface is easily derived from the TTL logic or microcontroller input/output.

To protect the display, an alternating voltage continuously changes the electric potential on the backplane. Segments are turned off by matching the voltage on the front plane to that of the backplane, while applications of an opposite voltage turn the segment on. Refresh rates are usually set between 30 and 120 Hz, depending on the specification.

Most high-density, high-contrast displays employ a type of liquid-crystal molecule that is often said to be "supertwisted." All liquid crystals exhibit some degree of twist. The amount of twist and how the molecule responds to an electric field affects its optical characteristics. When placed in an electric field, liquid crystals align so that their optical axis is parallel to the field. The realignment changes the crystal index of refraction. Because the molecular realignment is caused by a field, not by ion flow, current consumption is extremely low. Typical consumption is 0.5 to 0.6 ∝A/cm2 at 3 Vac.

Many LCDs that are not highly multiplexed use the so-called twisted nematic effect. The liquid-crystal material is between two sets of polarizers and electrodes. Each polarizer is 90° out of phase with the other. Inactivated liquid-crystal molecules have a 90° twist; activated crystals are straightened by the electric field. The degree to which a crystal straightens is a function of the field strength or activation voltage.

When the display is off, light passing through the first polarizer is twisted 90° by the liquid crystal. This allows the light to pass through the second polarizer. But an activated display does not rotate the light. Light passing through the first polarizer is at right angles to the second polarizer, and is therefore blocked. In large displays, the liquid crystals are used to form lines of dots. Multiple lines form a dot matrix. To address the thousands of dots that form a display, a multiplexing scheme is used.

The number of lines that can be directly multiplexed is limited. The problem is that as the number of addressed lines increases, the rms of the activation voltage available for each pixel falls off dramatically. For example, for a 100-line multiplexed display, the rms voltage for a selected pixel is only 10% greater than an off pixel. Consequently, the crystal does not twist very much, and a narrow viewing angle results.

Several methods are used to solve this problem. One involves subdividing the dot-matrix display into two or more smaller ones. For example, a single 640 X 400 display may be addressed as two 320 X 400 displays. While this technique provides a considerable increase in optical performance, it comes at the cost of increased drive circuit complexity.

A second, and perhaps more elegant, solution is to increase the steepness of the liquid-crystal brightness versus voltage curve. This curve is a function of how far the optical angle of the crystal tilts for an applied voltage. To increase the amount of tilt, several liquid-crystal parameters can be changed. Decreasing the crystal bend/splay elastic constant ratio, or the dielectric parameter=/normal increases the tilt. But even more dramatic results can be obtained by increasing the crystal twist angle from 90 to 270°. This degree of twist makes the slope of the optical tilt angle versus voltage curve become infinite. At twist angles larger than 270°, hysteresis becomes apparent. Crystals with a large degree of twist are often referred to as supertwisted.

To obtain a twist angle greater than 90°, the liquid crystal is doped with another optically active molecule that has either right or left-handed form. The "handedness" of the dopant twists the crystal. A 270° twist angle gives optimum contrast, but high manufacturing costs and visual distortion have been problems. Some vendors offer displays with crystals that are close to the optimum angle, for example, a 200° twist.

Using a backlight dramatically improves the visibility of LCDs, even when ambient light is good. But backlighting can also significantly increase display cost.

Plasma displays:Plasma-based devices are one of the most widely used types of flat-panel displays. Available as either dot matrix or alphanumeric units, most commercial displays have luminance between 10 to 100 fL, a viewing angle of about 120°, and contrast ratios of 20:1 or better. Additionally, they have a lifetime of about 20,000 hr, which is considered long. The disadvantages of plasma displays are their low luminous efficiency and high operating voltage.

The basic construction of a plasma display consists of rare gas (for example, argon or neon) sandwiched between two electrodes. When the potential between the electrodes increases to a threshold voltage VT the gas begins to glow. The gas continues to glow even when the potential drops from VT to a lower potential called the sustaining voltage Vs. This quality has generated a great deal of interest in plasma displays because it reduces display memory requirements. A pixel is turned on by addressing it with the difference VW = VT - VS, where VS is the potential of a strobe signal applied to the display. After VW is removed, the pixel continues to glow. This glowing reduces the need for refresh memory. Typical display densities of these devices are 128 X 32 and 280 X120 pixels.

Electroluminescent:There are two different types of EL displays, thin film and thick film. In both types, a phosphor emits light in the presence of an electric field. The monochrome phosphor is a blend of ZnS:Mn, which emits amber light. There are significant differences between the two EL types, including manufacturing techniques, optical qualities, and display lifetime. Driving waveforms are also different.

In thin-film displays, the light-emitting layer is only about one-half micron thick; total thickness of the phosphor and conductors is about 1 ∝m. Driving voltages are ac. On the other hand, thick-film displays have a phosphor layer about 25 ∝m thick and use dc voltages.

TFEL displays also contain layers of electrodes and insulators. Manufacturers are constantly seeking ways to improve these components because of their effect on the display's optical characteristics and life.

The electric field of a thin-film display is about 2 MV/cm. Because of the high field, any phosphor imperfection causing a short circuit between the electrodes can destroy the device. This possibility is prevented by current-limiting layers (insulators) sandwiched between the phosphor and electrodes.

On one side of the phosphor, both the insulator and electrode are transparent, allowing light through to the viewer. The insulator is one of the keys to display performance and reliability. Different manufacturers use different materials, usually an oxide of Al, Ti, Si, or Y, although BaTaO compounds are also used. The best insulators are insensitive to moisture and adhere well to the electrodes. Indium tin oxide (ITO) commonly forms the electrodes. Because ITO cannot handle high currents, driving voltages must be high (about 200 V) instead. Together, the phosphor insulators and electrodes form a capacitor in both thin and thick-film displays.

Ac TFEL devices perform better than dc thick-film displays in several ways. One is useful life. While dc devices work for about 10,000 hr, ac TFEL displays have a mean time to failure of about 40,000. The reason ac devices have a longer life is that ac inhibits phosphor migration and breakdown.

One difference of dc EL displays is that they are screen printed, making them simpler to produce. Ac TFEL fabrication uses a photolithographic technique similar to that used to make integrated circuits. Photolithography provides finer resolution than screen printing.

The most important factor determining the viewability of any display is its contrast ratio. Contrast ratio is the on brightness of the display (the amount of light emitted) divided by the off brightness. The larger the contrast ratio, the easier the display is to read. Generally, a contrast ratio greater than seven is recommended for full and half-screen displays used in a 500-lux ambient. Typical indoor work ambients range from 250 to 1,250 lux.

Other parameters affect display brightness and, hence, contrast ratio. Fill factor is one. It is determined by dividing the display's total area by its lit area. Fill factor is always less than one because there are spaces between individual pixels. Fill factor is an important consideration when choosing a display because the eye integrates brightness over the total display area. For example, if an individual pixel has a brightness of 25 fL, but the display fill factor is only 25%, the display itself only has an average brightness of 6.25 fL.

Most manufacturers quote pixel, not average display brightness. The waveform used to drive an EL display also significantly affects brightness. In both thin and thick-film EL displays, brightness is proportional to drive voltage, pulse width, and frequency.

Drive waveforms for TFEL displays must be synthesized carefully. One past problem is that asymmetrical driving methods, used in early devices, made the display retain a pattern. An asymmetrical waveform has pulses of different amplitudes, for example 190 V and -210 V. The net result was a dc bias on individual pixels, causing them to be brighter than unbiased pixels.

Display manufacturers also feel that pulse-timing relationships may contribute to afterimages as well. The evidence of this is that the center of TFEL screens show little effect on differential aging. The reason is that the screen acts as a delay line. At the screen's center, propagation delays tend to make the driving pulses evenly spaced. But at the screen's top or bottom, one polarity or the other dominates a given time period and causes differential aging.

Eliminating asymmetries in the driving waveforms prevents such effects. One method is to drive the rows with opposite polarity waveforms in alternate scanning times. Essentially, these driving waveforms are mirror images of each other and, thus, symmetrical. The actual method is called line-at-a-time matrix addressing. This means that the image is written on the screen line by line.

Since EL uses the basic video signals found in CRTs, off-the-shelf CRT controller chips can provide basic monitor control. These ICs provide all basic control signals plus the interface logic required to handle display refresh RAM and the CPU.

One factor that has hindered the development of full-color TFEL displays has been the poor performance of blue phosphors. The basic problem is that phosphors cannot produce light at this wavelength efficiently. However, new phosphors and deposition processes have recently been developed that emit blue light strong enough for use in EL displays. When used together with red and green phosphors, a full-color display is possible.

Two different methods are used to fabricate full-color red, green, blue (RGB) EL displays. The first uses a layered phosphor structure where phosphors of different color stack on top of each other. Here, thin-film layers are transparent and light from one layer shines through another. This type of structure requires the front EL phosphor to have transparent electrodes. One problem with this structure is that transparent-bottom electrodes are less reliable than opaque electrodes used in monochrome displays. This is one of the difficulties that have make stacked TFEL displays impractical.

The alternative to stacking phosphors is to split the pixel (as in CRTs) and place them side by side. This is referred to as patterned phosphor structure. Patterned structures require extra photoprocessing to delineate the phosphor stripes. The phosphor stripes are referred to as subpixels.

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