The Japan-based chip and epiwafer manufacturer has developed a fine-line electrode structure to improve light extraction.
Hitachi Cable, Ltd. has announced the development of a high-power red LED chip offering a maximum luminous flux of 55 lumens, at a current of 500 mA. This luminous flux was enabled by increasing the size of the LED chip and use of a fine-line electrode structure.
One of the methods of improving per-chip light output is to increase the chip dimensions. However, larger chips increase the difficulty in distributing a uniform current across the entire light-emitting layer. On the other hand, if large electrodes are positioned in the upper layer of the chip for a more uniform current dispersion, light from the light emitting layer will be blocked, reducing the light extraction efficiency (see Fig. 1: "Problems caused by enlarged chip size").
To resolve these problems in developing a new LED chip, instead of using larger electrodes, Hitachi Cable has utilized two pad electrodes for receiving power, a backbone electrode connecting the two electrodes, and multiple fine line electrodes that extend from the backbone electrode on the upper chip layer (see Fig. 2: "Structure of new high-power red LED").
By employing fine line electrodes, Hitachi Cable has succeeded in achieving uniform dispersion of current across the entire chip surface without blocking light from the light emitting layer, thus attaining a maximum luminous flux of 55 lumens in a large LED chip measuring 1 mm by 1 mm.
This level of output is equivalent to the combined output of 21 metal-reflector LED chips manufactured by Hitachi Cable. These 0.33 × 0.33 mm chips have an output of 2.6 lm at 20 mA.
Hitachi Cable expects the new LED chip to find applications in display applications, as well as devices such as projectors. The company says it will continue to pursue active development of higher-power LED chips while expanding its lineup of high-power LED chips, including yellow and infrared LED chips.
Hitachi Cable manufactures and supplies AlGaAs (aluminum gallium arsenide) and AlGaInP (aluminum gallium indium phosphide) epitaxial wafers, which can be used to manufacture red LED chips. The company has also developed high-brightness red LED chips that form a metal reflector (MR) under the light-emitting layer. This product is currently being supplied to LED package manufacturers and other customers.