Light me upAn emissive material has allowed light emitting diodes to shine brightly in the world. Steve Bush finds a new world opening up to LEDs - Jean Paul Gaultier take note... They used to be things that indicated that the power had been turned on, then they made pocket calculators possible.
Now they are items of fashion wear and may be the future in motorway signs.
What am I talking about? LEDs.
These solid state light sources have been around for over 25 years. The way they work is through the recombination of holes and electrons in the region of a p-n junction. The recombination releases energy in the form of a photon, which is visible if the band gap of the semiconductor is high enough.
Common semiconductors like silicon and germanium have low band gaps, so LEDs use more exotic III-V, high band gap, materials like gallium arsenide and indium phosphide. The basic structure of a standard red LED is an n-type GaAs substrate with a GaAs[sub1-X]P[subX] layer epitaxially grown on top.
The active layer needs to have 40 per cent phosphorous and 60 per cent arsenic. This material does not match the lattice of GaAs, so a buffer layer with an increasing gradient of phosphorous is built up before the active layer is deposited. A 3[mu]m p-diffusion into the top of the active layer forms the junction and a metal electrode is applied in the middle to take the gold bond wire.
Light is emitted through the top surface. Any that spills out sideways is directed upwards by the cup-shaped electrode that the chip is mounted in. Nearly half the light emitted goes back into the chip, where a proportion of it is reflected out again by the opaque substrate.
The plastic encapsulation has a lens formed into the end that focuses the light and determines the viewing angle of the device.
LEDs made like this are fine for simple indicators and indoor displays, but a new emissive material has had to be developed to make LEDs that can be used in outdoor displays.
The enabling technology for signs and other high brightness applications is AlInGaP (or InGaAlP, depending on the manufacturer). This is a four element compound based on aluminium, indium, gallium and phosphorous
Albert Marshall, III-V material development engineer for Temic, said: "The theory behind quarternary compound LEDs has been known for some time. But they could not be made with the existing vapour phase epitaxy techniques. The breakthrough that made them manufacturable was the availability of very pure metal-organic (M-O) compounds, such as tri-methyl aluminium, which enable chemical vapour deposition (CVD) to be used."
In the AlInGaP structure, the emissive structure has three layers forming the p-n junction. The top one is p-type and the bottom one is n-type. These layers are often called cladding layers.
The middle layer acts as a deep potential well which traps electrons and holes, dramatically increasing the likelihood of them combining in this region.
This middle region is shown as p-type in the diagram but can be undoped.
AlInGap is not a great conductor of electricity. If the top electrode alone was deposited on the active layer the only emission would be under the electrode. A current spreading layer, the conducts well, but is transparent to the emitted light is interspersed between the active layer and the electrode. In the diagram the current spreader is GaP.
A further increase in efficiency can be made if the substrate can be made more reflective.
One way to achieve this is to add a Bragg reflector between the substrate and the active layer triplet. Temic, for instance, use alternating layers of AlAs and GaAs for the reflector.
Toshiba's high intensity range use yet another trick to further increase efficiency. Light that is emitted under the electrode is wasted, so it adds and insulating layer to the substrate, under the electrode, to force current to flow through unmasked regions.
One extra trick to increase the proportion of light that escapes the device is to roughen the chip surface. This reduces reflection back into the chip from the interface.
AlInGaP is applicable to all LED colours between red and yellow-green, but not to pure green and blue.
Martin Lister, a spokesman on LEDs for Hewlett Packard, said: "The green needed for traffic lights and specified on some car dash boards has a wavelength between 555nm and 557nm is often called Lincoln or emerald green."
Green LEDs are still based on GaP, efficiency gains are being made by careful attention to the internal structure of the chips.
Blue LEDs were initially, and still are, made from SiC. Pakull Thomas, who is Temic's LED market segment manager, said: "We are developing gallium nitride on silicon carbide technology which is twenty times brighter than straight silicon carbide. It should be available in the second quarter of this year."
All this technology has enabled LEDs to find new markets and push incandescent sources out of some of their's.
Probable the most unusual LED application at the moment is in children's footwear. LA Gear, the US-based fashion company, produce trainers which light up as the child steps. Lots of fun for kids and a bit safer at night as well. For those of you who have seen these and wondered how they work, the bad news is that the power source it is not some exotic piezo electric device, but a battery and mechanical pressure switch. The good news for those who have bought them for their kids recently is that the batteries can be changed in the later models.
LEDs are now bright enough to be considered for information displays and other applications which have been the domain of incandescent bulbs.
Eddy Terris is managing director of illuminated sign maker Techspand. He said: "We made some of the variable speed limit signs on the M25 - A3 junction in surrey. We chose LEDs for three reasons. Their life is far longer than incandescent bulbs, they use less power and the optical performance is better." But he added: "The optical performance is not greatly better the older technologies, and we still use quartz-halogen bulbs for white light."
Traffic lights are another potential application. Temic make LEDs for traffic lights in Brazil. But the market is unlikely to take off until an efficient and cost-effective pure green LED is available.
As well as roadside applications, LEDs have made there way into vehicles. Peter Gray, a senior marketing engineer at Toshiba, said: "LEDs are now used in after-market high mounted stop lights and in amber side lighting strips on lorries."
DoT rules allow LEDs to be used as indicators and stop lights as long as they have been approved. But there are still technical issues to be resolved. Pakaull Thomas of Temic said: "High mounted stop lamps are a growing market, but LEDs are not really bright enough for other uses yet. Temperatures of 110[deg]F may be experienced in car lighting clusters, which is too high for LEDs at this time. I don't think we will see many direction indicators and brake lights using LEDs until 2000 or later."
AlInGap technology has dramatically increased the brightness of most of the spectrum of LEDs. So much so that they can now be used in many outdoor applications including traffic signs.
Pure green and blue LEDs cannot respond to the AlInGaP treatment, but other developments are increasing the brightness of these as well.