Improvements still needed to make LEDs more efficient
Two recent developments are set to make LEDs more efficient. One, from Osram, deals with the core technology in the chip, while the other, from Philips, addresses packaging to provide electronics designers with more flexible, better performing products.

Osram has grown, literally, a solar light in the infrared spectrum and claims to have made huge strides along the path of energy efficiency, although the company's Markus Broell concedes the 25% efficiency improvements recorded in the lab will not be replicated in full production models. Broell, project manager for the development of infrared LED chips at Osram Opto Semiconductors, believes the research team has achieved 'something special'.

The first product to use the new technology will be an 850nm wavelength infrared (IR) device. For applications in security, this wavelength provides a good balance between being invisible to the human eye and being detectable by sensors.

At the top end of the IR spectrum 700nm a red glow can easily be seen and silicon based sensors are at their most sensitive. At the other end of the spectrum, this sensitivity has faded, but the light becomes completely invisible. An intruder looking for a security system using an 850nm LED would be able to see it just but would need to know where to look and to be looking straight at it.

With production set to be underway by this summer, the new die technology will be used in the light source for the high efficiency versions of the company's Dragon range of IR LEDs. The efficiency improvements, according to Broell, are down to the thin film technology used. "We have optimised the current distribution over the chip," he explained, "and have minimised absorption losses at both interfaces not only the bottom interface of the active layer, but also the top interface.

By reducing the losses and enhancing the 'outcoupling', we have achieved these performance increases." Outcoupling is a technique which enables more light to escape, rather than be lost through internal reflection.

The material used for the crystal is aluminium gallium arsenide with a little bit of indium and traces of doping materials like carbon and tellurium and the film is just 6um thick. The crystals have been grown epitaxially at the company's Regensburg labs in southern Germany.

The wavelength is determined by some of the layers in the centre of this layer stack. These layers, only a couple of nanometers thick, are called quantum wells and convert the electrons and holes within a semiconductor into a photon. So, in the quantum world, the electron drops to a lower energy state and then emits a photon.

Broell added: "The basic principle is whenever you make an LED, you want to convert the electron pulse as efficiently as possible into photons, and you want to get the photons out of the crystal as efficiently as possible. We have increased the efficiency in both of these areas; producing more photons, reducing absorption and increasing the efficiency of getting the photons out of the crystal. External Quantum Efficiency is about how many of these photons can escape the crystal before being lost in absorption again."
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