LEDs are still popular and improving after all these years: Part 3
LED Life Expectancy
LEDs have a MTBF (mean time between failures) usually in the range of 100 000 to over 1 000 000 hours. This is a long time for continuous operation, considering a year is 8760 or 8784 hours. In practice, the useful measure of LED lifetime is its half-life that is an LED is deemed to have reached the end of its life when the light output falls off to half the original.
When current flows through an LED junction the current flow is not uniform, resulting in small temperature differentials within the chip. These temperature differentials exert stress on the lattice, causing minute cracks to occur. These lattice defects accumulate with use, and reduce the photon conversion efficiency of the chip, so reducing light output. The attrition rate varies according to the LED material, temperature, humidity, and the forward current.
Blue and White LEDs
There are essentially two technologies for generating white light from LEDs. One way is to mount a red die, a green die, and a blue die very close together within a package, and mix the light outputs in the correct proportions to achieve white light. The problem with this approach, ignoring the technical issues of setting the correct LED drive levels, is the cost of 3 dice. Nonetheless, tricolor LEDs are popular for LCD backlights in consumer applications because the user can set the backlight color to any hue desired.
The cheaper approach, pioneered notably by Nichia, involves including a phosphor with the blue LED that absorbs some of the blue light and fluoresces in a second color to achieve a near-white. Some early white LEDs using this technique showed a noticeable blue tinge, but the most recent developments are excellent and can be seen in the emerging full color PDAs and cell phones.
Recent Applications for LEDs
LED processes changed rapidly in the 1980s with the emergence of high efficiency GaAlAs and ultra efficiency InGaAlP LEDs (Table 2). In a short space of time, the quantum efficiency of LEDs was approaching several percent, all primary colors (RGB) were available, and reliability was at least as good as the other display technologies. Surface mount LEDs are available in single color (including white), bi-color (usually red and green), and tri-color (Figure 15) and these have proliferated in backlights for smaller LCD panels, equipment panels, and indoor message boards.
Outdoor message boards using LEDs instead of filtered incandescent lamps use clusters of LEDs grouped together close enough so that the light outputs merge to create a typically 25mm square pixel (Figure 14). These message boards (or variable message signs) are used for advertising displays and traffic signs.
Another rapidly growing market is traffic lamp replacement. Incandescent traffic lamps draw somewhere between 75W and 150W, depending on size (20cm or 30cm) and color (due to differences in the transmissivity of the red, green, and orange filters used). LED traffic lamps draw around 7W - 15W, and can be replaced every 5 years instead of every year for incandescent.
Table 2. LED Processes
| Light Emitting Layer | Timeline | Comments |
|---|---|---|
| GaAsP (Gallium arsenide phosphide) | 1960s | Original low efficiency red using liquid phase epitaxy |
| GaP (Gallium phosphide) | 1970s | High efficiency red |
| GaA|As (Gallium aluminum arsenide) | 1980s | Single and double heterostructure processed using vapor phase epitaxy increase efficiency |
| InGaA|P (Indium gallium aluminum phosphide) | 1990s | Metal organic vapor phase epitaxy |
| InGaN (Indium gallium nitride) | 2000s | Ultrabright green and blue |
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| Fig. 14 LED cluster pixel for outdoor message boards | Fig. 15 Surface mount LEDs from ever light |
Future Applications for LEDs
Current ultra brightness LEDs exceed the light output of incandescent and halogen lamps and are not subject to the maintenance requirements (a life of a few thousand hours at best) associated with filament lamps. Also, LEDs are easily dimmed using PWM an other techniques. So the goal of the LED process developers is to build a very high brightness white LED which is economic enough to be used for domestic lighting. Right now, there is interest in high efficiency, long-life lamps by hotels and factories because not only is electricity for lighting a significant expense, but there is also the labor cost in actually replacing bulbs to consider also.
Comparison of Display Technologies
Liquid Crystal Display (LCD) – Reflective
Technology – An LCD uses the properties of liquid crystals in an electric field to guide light from oppositely polarized front and back display plates. The liquid crystal works as a helical director (when the driver presents the correct electric field) to guide the light through 90° from one plate through the other plate.
Advantages:
- Small, static, mono panels can be very low cost
- Both mono and color panels widely available
- Static panels offer lowest power/voltage display
- Reflective panels in general are low power
- Very easy custom segment shapes, sizes
- Reverse backlit mono panels are attractive
Disadvantages:
- Backlight adds cost, and often limits the useful life
- Requires AC drive waveform
- Fragile unless protection added
- Can have narrow temp range (0°C – 50°C)
- Temperature compensation usually required
- Can have narrow viewing angle
- Low yields raise cost for larger (17”+) displays
Light Emitting Diode (LED) - Emissive
Technology – LEDs are photon emitting semiconductors which emit light due to the injection electroluminescence effect. The wavelength of the emitted light varies primarily due to the choice of semiconductor materials used, and is commonly in visible spectrum or infra-red.
Advantages:
- Lowest cost red or green emissive indicator
- Available in very small sizes
- Very bright versions available (higher cost)
- Red and green types work from 3V supply
Disadvantages:
- LED is point source, so light shaping required to make segment shapes
- White and blue LEDs expensive, need >3.6V supply
- Can have narrow viewing angle
- Color and efficiency vary with temperature and current
- Care required to achieve 50khrs+ life
Organic LED and Polymer LED - Emissive
Technology – These displays use organic electroluminescent materials deposited on a glass or flexible substrate. Devices based on small molecules are usually referred to as OLEDs. Those based on large organic "polymer" molecules are usually called PLEDs. Light is generated by injection electroluminescence, like LEDs. The choice of organic material sets the emission color. OLED pixels are capacitive (10s to 100s of picofarads) leading to significant switching losses for large displays with high multiplex ratios.
Advantages:
- Moderate cost for small (<4") color panels
- Wider viewing angle than LCD
- Faster element response than LCD
- Emissive, unlike LCD color panels RGB and mono displays
- Can be built on a flexible substrate
Disadvantages:
- 6V to 16V operating voltages
- Differential aging effects limit life
- Power consumption high for matrix panels >128x64
Vacuum Fluorescent Display (VFD) – Emissive
Technology - The VFD is a vacuum tube using hot filaments to generate thermo electrons, A grid (static display type) or multiple grids (multiplexed display type) control and diffuse the thermo electrons, which are attracted to one or more high voltage phosphor coated anodes, which then emit light. The anodes are at the back of the display, so the emitted light passes through the grid(s) and filaments and the display front to be seen by the user. The filaments are not run hot enough to be usually visible.
Advantages:
- Wide operating temperature range
- Long (40khrs+) life
- Wide viewing angle
- Very bright, attractive, typically green display
- Very easy custom segment shapes, sizes
- Different colored segments easy
- 12V grid/anode voltage versions available
Disadvantages:
- Filament supply (±8% typ. tolerance) required
- 10V to 60V grid/anode operating voltages
- RGB displays available, but expensive
- Phosphors other than green limit display life
