DISPLAYS - Better mounts produce brighter blue LEDs

KAGOSHIMA-Researchers at Matsushita Electronics Co. have developed a new mounting technology for use in the mass production of brighter, more reliable blue and green light-emitting diodes (LEDs). The LEDs are fabricated from gallium nitride on a sapphire substrate and produced by Toyota Gosei Co. (Tokyo).

Conventional LEDs use clear electrodes, such as indium tin oxide, which absorb some of the light. The absorption could be reduced by making the electrodes thinner, but this solution lessens the efficiency of the electrodes.

A sapphire substrate is virtually transparent to visible light, so LED luminance could be increased if light were emitted through it instead of through the electrodes. However, this solution requires the difficult manufacturing step of installing the LED chip upside-down, with the electrodes affixed to its base.

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Matsushita has developed a mounting technology that allows large numbers of LED chips to be directly affixed to their bases. The electrodes are highly reflective, directing light through the sapphire substrate. The finished LED has twice the luminance of conventional devices (see figure).

The element used for the base functions as a zener-or voltage stabilizer-diode. When a reverse voltage is applied to a zener diode, current flows when the voltage reaches a certain threshold value. If this diode is connected in parallel with the LED (which is opposite in sign), the LED is protected from static electricity. Gallium nitride LEDs have been sensitive to static electricity, but this method produces a resistance to static charge of more than 1 kV (both positive and negative)-30 times better than previous performance-and makes assembly much easier. In addition, the base element provides good heat dissipation.

Courtesy O plus E magazine, Tokyo