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LED Background

Most LEDs with output in the blue end of the spectrum are formed by growing GaN epitaxial layers with a total thickness of about 100 µm on top of a sapphire substrate. These sapphire substrates are usually about 2 inches (50 mm) in diameter, while the individual LEDs might cover an area of about 300 µm x 300 µm. Thus, a single sapphire wafer can contain 20,000 or more devices. After epitaxy, the wafer must be cut to separate out (singulate) the individual LEDs for packaging.
 
Sapphire is used as the substrate material for several reasons. First, it provides an excellent lattice match for the GaN, and, second it is transparent to LED output wavelengths, which is important since some of the light emitted by the LED partially passes through the edge of the sapphire substrate. Finally, sapphire is a fairly good thermal conductor, which helps in heat sinking the LEDs.


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