LEDs poised to brighten our lives — Part 1

It is perhaps surprising that in the nearly 50 years since LEDs first saw the light of day, only now are these little devices becoming so widely used.

However, nowhere is the future brighter than in Taiwan where many companies are developing some of the latest technologies to bring these devices into ever more corners of everyday life.

Increasingly warm relations with Big Brother China, and an already vibrant home and export electronics industry and government plans to make Taiwan the crossroads for future world distribution, have put this little island into the box seat for development and the export of high-tech devices including LEDs and ICs.

There is little doubt, too, that a growing awareness of global warming and the fact that thermal power stations are one of the biggest threat to the ozone layer have sharpened attention on the low-voltage, low-current demands of LEDs; especially for lighting, which is seen not only as one of the biggest electricity consumers but also one where these diodes could make a major impact.

Unlike many western countries, the Taiwanese government is seriously active in encouraging its electronics industry. This has come about hand in hand with the improving relations with mainland China.

Last year’s Taiwan Business Alliance Conference, although focused on increasing trade with China, also looked more generally at trade. It sees the country’s geographic location as making it a ‘linchpin’ in the shipment of goods such as electronics whether from south-eastern China to America or from north-east Asia to Australia and New Zealand.

Big plans to improve the island’s access to the world include a rapid transport network, a free trade harbour in Kaohsiung and developing Taoyuan City as an ‘air city’. These moves, coupled with future direct air and sea links between the two sides of the Taiwan Strait and a series of tax reforms and deregulations, are all designed to attract foreign investment and native research to further the island’s competitiveness, especially in electronics and especially in LEDs and semiconductors/ICs.

To particularly stimulate the LED industry, the government has introduced a whole series of lighting-related policies. The ‘Lighting Energy Saving Promotion Plan’ and the 'LED Road Light Demonstration Project’ are two initiatives aimed at replacing traditional incandescent lighting with LEDs and to provide the use of LEDs in exit and escape signs in domestic and office buildings.

All this action at home, plus supporting booming exports, means there are about 200 manufacturers involved in the LED production chain in the country, covering epitaxy in the upstream, die production in the mid stream and packing in the downstream.

This makes Taiwan the second largest LED manufacturer in the world with a gross output value in 2007 reaching AU$2.124 billion and representing about 21.6% of the world’s total output value. Last year’s annual figure was predicted to reach AU$2.38 billion, a 12.1% annual growth rate.

However, as LEDs prepare to take over the world, it’s worth looking at the history of these devices.

They were slow starters in the dim 60s. In the beginning they combined gallium arsenic and phosphorus to produce a 655 nm red light which because of its dimness was suitable only as an indicator light.

Gallium phosphide lights followed and although these were quite efficient in terms of voltage and current demand and brightness, their wavelength at 700 nm was in a region of human vision that is quite hard to see so their main use was still as indicator lamps.

It was also found that the efficiency of these LEDs was best at low currents. As the current was increased the efficiency decreased, making them unsuitable for use as outdoor messaging where the devices were multiplexed to achieve readable light levels.

However, the diodes found commercial use but only in a limited range of applications.

Through the 70s, more colours and wavelengths were developed with gallium phosphide providing green and red, gallium arsenic and phosphorus, orange of high intensity, red and yellow. All these are in current use.

As more types became available, so they started appearing in calculators, test equipment and digital watches. However, these early LEDs had a high failure rate due to their assembly, which was mainly manual.

This resulted in all sorts of failures from short circuits to forward and reverse voltage leakage so that for a time the diodes had a reputation of unreliability.

The introduction of a new material, gallium aluminium arsenide (GaAIAs), brought a surge in the use of LEDs. Brightness was more than 10 times that of earlier types, helped by the adoption of hetero junction type structures. The operating voltage was lower giving power savings and the devices could easily be multiplexed or pulsed.

They were taken up for use in messaging and outdoor signs and designed into bar code scanners, fibre-optic transmission systems and medical equipment.

Unfortunately, GaAIAs was not the total answer to the perfect LED. The devices were available only in red at 660 nm and their degradation was higher than earlier types. Some GaAIAs devices showed a decrease in efficiency of up to 50% after only 60,000 hours of operation.

This was particularly the case where they were used in high-humidity/high-temperature situations. Also, it was found that yellow, green and orange showed only a minor improvement in brightness, which was put down to better crystal growth and optic design, not better electronics.

The next major development came in on the back of laser diode technology that was produced for bar code readers, measurement systems and storage devices.

Indium gallium aluminium phosphide (InGaAIP) as the luminescent material allowed output colours to be designed by adjusting the size of the energy band gap so that green, yellow, red and orange LEDs could all be produced using the same basic technology. And, as a bonus, light output degradation was no longer an issue.

Toshiba took the LED story a major step forwards when it developed the metal oxide chemical vapour deposition (MOCVD) growth process to produce a device that reflected 90% or more of generated light travelling from the active layer to the substrate back as useful light output. This almost doubled LED luminance.

A further development introduced a current blocking layer into the LED structure that channels the current through the device for better efficiency.

Blue LEDs using gallium nitride (GaN) and silicon carbide (SiC) with brightness levels of 1000 mcd at 20 mA and other high-intensity colours have given improved traffic light illumination and many pieces of equipment that are now cheaper to run, cooler to run, are more reliable and have a longer life.

Taiwan Trade Centre, Sydney