The first true ancestors to the Indium Gallium Nitride (InGaN) LED evolved last decade. These took the form of blue LEDs utilizing Silicon Carbide (SiC) as the active, light-emitting material. These early LEDs were characterized by very low light output, less than 2cd/m². The next generation of blue LEDs relied upon SiC as a base layer only and employed Gallium Nitride (GaN), grown directly on the SiC substrate, as the active, light-emitting epitaxial layer. This process initially increased light output by a factor of eight. The final iteration saw the introduction of Indium (In) to the epitaxial layer to form InGaN. This development further boosted light output by a factor of five - a full 1300% increase in intensity over the first SiC LEDs. Today, through advances in process, packaging and thermal transfer technologies, light output continues to evolve.

Besides increasing the intensity of blue, and by extension, white LEDs (since all white LEDs use a blue chip in conjunction with a light converter, or phosphor), the InGaN process has replicated two new colors: verde and true green. These unique colors, alongside InGaN’s high intensity and inherent reliability, has greatly increased their proliferation into applications once reserved solely for incandescent lighting: traffic signals, realcolor displays, message boards, moving signs, dashboard backlighting, battery flashlights and toys.

While the InGaN process produces the brightest light output across blue, verde, true green and white, it is important to understand that the wavelength of the light emitted is strongly dependent upon the forward current driven through the device, and that in order to avoid shifts in color, careful consideration must be paid to dimming strategies. This application note, then, will examine methods for dimming InGaN LEDs with little or no effect on wavelength.

Some years ago, the color range of Light Emitting Diodes (LEDs) on the market was limited to the red to green spectrum. Then, blue LEDs were developed and introduced into the market. These blue devices made it possible to build so called “single-chip white“ LEDs, using a yellow converter material in combination with a blue die. Most of the blue and white LEDs use Indium Gallium Nitrite (InGaN) as an epitaxial layer. The wavelength (chromaticity coordinates) of the generated light of these InGaN-based LEDs shows a strong dependency on the driving current. This special property of InGaN based LEDs must be considered well in advance for new application solutions. This application Note is intended to enable the reader to avoid some common design mistakes when using InGaN-LEDs.