LEDs - or Light-Emitting Diodes - Are Semiconductors That Generate Narrow-Spectrum Light

LEDs - or Light-Emitting Diodes – are semiconductors that generate narrow-spectrum light when electrically biased in the forward direction of the p-n junction. This effect is a form of electroluminescence.

A single LED is often a small area source, usually with extra optics added to the chip that shapes its radiation pattern. The colour of the LED is a result of the composition and condition of the semiconducting material used and can range from infrared, to visible and near-ultraviolet.

In recent years, LEDs have come from only being little red dots that let users know if electrical appliances were switched on, to extremely bright light sources capable of lighting up a room or illuminating the road ahead of a vehicle.

Despite concerns over the UK lagging behind in LED technologies, industry insiders are confident Britain is on the cusp of a boom in the lighting sector. In fact, market watcher Strategy Analytics predicts the LED market will grow by 21 per cent.

Certain industry experts have claimed LEDs will take over the lighting market, while others are not as enthusiastic, claiming the technology is too advanced to be squeezed in a one-size-fits-all approach.

A new generation of light-emitting devices is set to become widely available. Photonic Lattice LEDs are able to be built in large numbers providing extremely high performance for projection and other applications.

Functioning like any other diode, an LED is comprised of a chip of semiconducting material steeped with impurities to create what is known as a p-n junction. Currents flow easily from the p-side – or the anode – to the n-side – or the cathode – but they do not move in the opposite direction.

Charge-carriers such as electrons and holes flow into the p-n junction from electrodes with different voltages. When an electron meets a hole, it drops into a lower energy level and released energy in the form of a photon.

The wavelength of the light generated, which determines the colour produced, depends on the band gap energy of the materials forming the p-n junction. The materials used for an LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.

LEDs will only work with positive electrical polarity. When the voltage flows in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows and no light is generated. LEDs can work on an alternating current, but they will only emit light with positive voltage causing the LED to turn on and off at the frequency of the AC supply.

White LEDs

High quality LEDs are now produced with efficiencies around 70 lm/W, compared to 12 lm/W for incandescent light bulbs and single die devices with over 100 lm.

White LED versions of usual lights are already produced from several manufacturers.

LED headlights for vehicles are planned, with German electronics firm Osram leading the way. Osram is developing a range of multi-die packages that are very close to generating enough light for vehicle applications.

In addition to vehicle lighting innovation, Osram and Seoul Semiconductor have joined forces to share patents for white LEDs.

Lighting specialist Lumileds will swap materials for white LED car headlights. Lumileds will replace liquid-deposited phosphurs to bonded phosphorescent ceramics for applications next year.

Coloured LEDs

LED technology has advanced to such an extent that coloured LEDs have now moved into applications that used to be the domain of filtered incandescent light.

One example of LEDs moving into everyday applications is in car rear light clusters where red LEDs began to be seen almost as soon as their efficiency allowed.

Car manufacturers were particularly keen to take up LED technology as the rear light cluster could be made significantly thinner thereby requiring less boot space and they could eliminate the capability of owners to change their own rear light bulbs.

OLEDs

Organic Light Emitting Diodes (OLEDs) are semiconductors where the emitting layer material of the LED is an organic compound. The emitting matter can be a small organic molecule in a crystalline phase or a polymer. Polymer materials can be flexible – such LEDs are called PLEDs or FLEDs.

OLEDs are lighter than other LEDs, and PLEDs have the added advantage of being flexible.

Practical applications of OLEDs include visual displays for portable electronic devices including phones, digital cameras and MP3 players. OLEDs have been used for larger displays, but their life expectancy is still too short to be practical.

LED developer Thorn Lighting is working on a project to design OLED materials and device architectures for large area applications which could mean the end of fluorescent tubes.

"The materials we are hoping to develop will give high quality light similar to the sun," project head Dr Geoff Williams said.

Industry experts believe although OLEDs aren't taking the market by storm, there is certainly a healthy demand for OLEDs. Some experts are still waiting for OLED TVs to hit the marketplace.

Advantages of LED

LEDS generate more light per watt than incandescent bulbs and manufacturers favour these in many battery powered or energy-saving devices. Also, LEDs can illuminate in their own colour without the need of colour filters that may increase production costs. LEDS can focus light on their own whereas incandescent and fluorescent sources need external equipment to collect and deflect light.

Disadvantages of LED

Calculated on an initial capital cost basis, LEDS are more expensive than other lighting technologies. LEDs cost more as they have a relatively low lumen output and the drive circuitry and power supplies needed are also more expensive. LEDs are also affected by the temperature of the operating environment. Operating an LED in extremely high temperatures may result in overheating, which could lead to failure of the device. Overheating is usually addressed by heatsinking the LED. Although LEDs are supposed to have a higher life expectancy than other lighting technologies, but some cheap LEDS can struggle to live to see a few months.

Related Links:

LED technology - Coloured LEDs

LED technology - White LEDs