Die Fabrication
The semiconductor dies that form the die matrix of the COB array are indium gallium nitride (InGaN) LEDs. The InGaN direct bandgap semiconductor is doped with acceptor impurities and donor impurities to a positively charged (P-type) layer and negatively charged (N- type) layer, respectively. These InGaN layers are grown on a sapphire, silicon carbide (SiC), or silicon wafer. The wafer material has a significant impact on the efficiency and thermal performance of the LED. Sapphire is the predominantly used die substrate material but its density of threading dislocations to epitaxial layers is much higher than SiC. This translates to relatively low internal quantum efficiency. And SiC's high thermal conductivity of 110 - 155 W/mK allows GaN-on-SiC LEDs outperform GaN-on-Sapphire LEDs in terms of thermal conduction capacity (Sapphire has a typical thermal conductivity of 46.0 W/mK). The epitaxial layers are typically stacked with a standard chip structure found in SMD devices. Lately there ha s been a trend to use the flip-chip structure to make a chip-scale package (CSP) for COB applications.
Depending on the light output of the COB LED package, InGaN diodes of various power ratings are used. The use of low power LED dies will inevitably increase wire bonding density and subsequently the cost and process complexity, and the use of expensive high power LED dies will compromise luminous efficacy and cause heat flux concentration. Therefore most InGaN LED dies incorporated in COB packages are usually mid-power chips in the 0.2W - 0.5W range.
