Tamura Corp and Koha Co Ltd developed a white LED using gallium oxide (β-type Ga2O3) and exhibited it at Lighting Japan 2013, a trade show of lighting devices, which took place from Jan 16 to 18, 2013, in Tokyo.
The LED consists of (1) a blue LED chip made on a β-type Ga2O3 substrate by using GaN-based semiconductor and (2) a fluorescent material. Compared with a case where a common blue LED chip formed on a sapphire substrate is used, the new LED realizes a higher optical output, the companies said.
At the show, Tamura and Koha showcased two kinds of white LEDs that use 0.3 x 0.3mm and 2 x 2mm blue LED chips, respectively. Their driving currents are 400mA and 6A. The brightness of the LED using the 2 x 2mm chip is about 500lm. And the companies aim to increase it to 2,000 to 3,000lm, which is difficult to realize by using a sapphire substrate, they said.
With a blue LED chip formed on a sapphire substrate, it is necessary to use the "horizontal structure," in which anode and cathode electrodes are horizontally arranged because a sapphire substrate is an insulator. On the other hand, a β-type Ga2O3 substrate has a high electrical conductivity and allows to use the "vertical structure," in which anode and cathode electrodes are formed on the top and bottom sides of an LED chip.
Compared with the horizontal structure, the vertical structure makes it easier to reduce element and heat resistances and to realize an even current distribution. The smaller the element and heat resistances, the smaller the amount of heat generated by an LED chip. Therefore, the new LED is suited to be driven by a high current.
Though GaN substrates are also suited for making high-output LEDs, β-type Ga2O3 substrates realize lower costs, Tamura and Koha said. Currently, GaN substrates are manufactured by using a vapor-phase epitaxial method called HVPE. On the other hand, β-type Ga2O3 substrates are manufactured by the melt growth method, which is theoretically suited for improving the quality of substrate and increasing substrate size.
The diameter of the β-type Ga2O3 substrate that has already been commercialized is 2 inches. The companies plan to prototype a 4-inch product within 2014 and commercialize it in 2015. They will also try to develop a 6-inch product.