Silicon Carbide Substrate Boosts LED Luminosity
The majority of contemporary LEDs are constructed from a combination of Indium gallium nitride (InGaN) and sapphire substrate. The architecture works well and has allowed LED manufacturers to offer products exhibiting efficacies in excess of 150 lm/W. However, the architecture does have some drawbacks which have encouraged chipmakers to seek other options.
One commercially successful alternative is silicon carbide (SiC), and LEDs based on the substrate have been on the market for two years. Now a new generation of the technology has been released that promises to double the luminosity of the current brightest single LEDs and cut lighting fixture costs by 40 percent.
This article looks at SiC technology and describes the latest chips based on the material to see how they compare with the previous generation and contemporary sapphire-substrate LEDs.
InGaN is the material of choice for the manufacture of today’s high-brightness white LEDs. The bandgap of the semiconductor is carefully manipulated such that the LED die emits blue photons, the majority of which are absorbed by the LED’s phosphor coating and re-emitted in the yellow part of the spectrum. The mixing of blue and yellow light produces a good approximation of white light.
Unfortunately, unlike the silicon used for most integrated circuits –– which can be cheaply produced leading to low-cost components –– InGaN is difficult to manufacture in large ingots. LED makers overcome this difficulty by using an epitaxial technique such as metalorganic-chemical-vapor deposition (MOCVD). This process overcomes the need to grow bulk InGaN, instead building up the material by depositing successive thin films on a suitable substrate.
The most common material for the substrate is sapphire (Al2O3). The mineral is cheap, durable, and a good insulator. Figure 1 shows a cross section of a sapphire-substrate LED.
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