LED Modern Applications and Future Advancements • Timothy Delf • Jason Brownlee • February 18, 2005 • ECE 3611 • Dr. Stanley G. Burns Focus of Report • LED Background • Structure and Concepts Regarding LEDs • Future Advancements & New Technology • OLED • White LEDs • Problems Facing LED Advancements Background • Available in multiple wavelengths & colors • Long life at 100,000 hours LED's are the light source with the longest life currently available in the world. • LED's operate as 12 and 24-volt DC systems • High luminescence efficiency • Based on the general properties of a simple twin-element semiconductor diode • LEDs don’t emit ultraviolet light, which degrades electrical components, and they emit minimal infrared light, the main component of heat Background (Cont.) • Extremely quick refresh rate • Exceptionally wide color palette • Potential for mind-boggling efficiency • Numerous Applications (cell phones, signs, stereos, etc.) Colors Available • Red • Amber • Yellow • Green • Blue • White Structure and Concepts Regarding LEDs • Heart of a light emitting diode is a semiconductor chip. • The diode semiconductor contains several very thin layers of material that are sequentially deposited onto a supporting substrate. • Diode chip is mounted in a reflector cup connected to a lead frame, and is bonded to the anode and cathode terminals of the frame through miniature bonding wires. • Entire assembly is then encased in a solid epoxy dome lens Structure and Concepts Regarding LEDs • A diode comprises a N-type material bonded to P-type material, with electrodes on each end. • Connecting the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end eliminates depletion zone. • Free electrons moving across a diode can fall into empty holes from the P-type layer. • Creates Photons - basic units of light. All diodes can produce “light” • Most just don’t produce visible light • Wavelength of the photon must be between 400 nm and 750 nm Even if a diode does produce visible light it must also meet these requirements to be effective • Must not absorb the photons emitted • Instead release photons outward • Plastic bulb to concentrate light (a) Diode absorbs photons (b) LED reflects photons (c) Bulb holds photons at top Future Advancements: OLED • Organic light-emitting diode-- display device that sandwiches carbon-based films between two charged electrodes • When voltage is applied to the OLED cell, the injected positive and negative charges recombine in the emissive layer and create electro luminescent light • Beginning to replace LCD technology in handheld devices (PDA, Cell phone, etc) Future Advancements: OLED- Advantages Over Liquid Crystal Display (LCD) • Don’t require a backlight • Faster response time • Smaller and thinner • Less power consumption • Cheaper to make Future Advancements: OLED • Doping or enhancing organic material helps control the brightness and color of light • Active matrix OLED displays stack cathode, organic, and anode layers on top of another layer – or substrate – that contains circuitry • Passive matrix OLED displays stack layers in a linear pattern, much like a grid, with “columns” of organic and cathode materials superimposed on “rows” of anode material Future Advancements: OLED • Future Advancements: White LED • Caused by adding phosphor to material used to create blue LEDs • InGaN + Phosphor • Useful for automobile headlights, etc • Eventually used for general illumination General Illumination: LED • Not possible yet • Needs greater efficiency • Perfection of light that is perceived as true white to human eye • Eventually could take over the incandescent and fluorescent light bulb Problems Facing LED Advancements • White LED lights include a lack of high efficiency at all colors, especially in the yellow/green where luminosity is the highest • Lack of efficient color mixing • LEDs dissipate heat by conduction rather than radiation Efficiency Problems • Some electron-hole combinations create heat-producing vibrations instead of photons • Some internal resistance also produces heat • Research on using different materials will hopefully solve this Problems with OLEDs • Color OLEDs still aren't as long-lived as they need to be; reds and blues tend to die after a few thousand hours, leaving plenty of green and not much else. • While OLEDs deliver positive qualities like high contrast ratio, wide viewing angle, and lots of brightness, the down side is that the paper thin- film material reacts immediately when it comes in contact with water, creating black spots on the surface. Conclusion • LEDs have a promising future • OLED display could take over LCD • True white LED becoming possible • Eventually replacing incandescent & fluorescent light bulbs for general illumination Questions?
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