Sanken unveils AlGaInP-on-Si LEDs
The new LEDs employ large-area chip designs that carry up to 40 times more current than the company s standard 0.3 mm x 0.3 mm format LEDs. The new power LEDs measure either 0.7 mm x 0.7 mm or 1.0 mm x 1.0 mm, and use currents of 400 and 800 mA to drive the 620 nm red LEDs to luminous outputs of 37 and 74 lm, respectively.
The forward voltage was not specified. Yellow LEDs with a peak wavelength of 590 nm deliver 21 and 42 lm of luminous flux under 400 and 800 mA operation, respectively.
By comparison, Sanken s conventional LED chips offer 0.74 lm in red and 0.42 lm in yellow, using a current of 20 mA. The new 1.0 mm x 1.0 mm devices represent a 100-fold improvement in output on its standard die, although the final output will depend on packaging and use, says the company.
Sanken has been developing blue GaN emitters on Si substrates since 1995, and earlier this year described developments with low brightness blue devices. The company employs silicon substrates to dissipate heat caused by the large currents used in the AlInGaP chips, which would typically be grown on GaAs substrates. Silicon substrates provide a higher thermal dissipation than GaAs, benefit from mature processing technology and are less expensive.
During the wafer bonding process, Sanken required high temperatures to bond the quaternary with the silicon substrate, which led to the flexing and cracking of the LED wafer as a result of the differing thermal expansion coefficients. In addition, the company had to circumvent the difficulty of lost light due to the absorbing nature of the silicon substrate.
To address these issues, Sanken added a thin metal layer between the epitaxial layer and the silicon substrate to decrease the bonding temperature and prevent LED wafer damage during processing. The company says the lower bonding temperature simplifies processing and improves productivity. The metal layer also acts as a high fractional mirror, improving the external efficiency of light by as much as 2.5 times.
Sanken will leverage its existing silicon power transistor assembly lines for the new LEDs, and is planning to further develop GaN-on-Si blue and white emitters using the wafer direct bonding technology.
