Digital Tech

How LED technology works: color LEDs, and white LEDs

A LED (light emitting diode) is an electronic component that emits light from an electrical excitation.

From car headlights to living room lights, LEDs are everywhere. Learn how they work, how they are made, and the reasons that make LEDs the light sources of choice for tomorrow. The parameters that distinguish the LEDs will also be reviewed.

A semiconductor, made from the stacking of layers of different materials and thicknesses, constitutes what we could call the motor of the component by creating a semiconductor junction. It is welded on a support that is both conductive of current and heat, then protected by a layer of transparent material, allowing the light emitted to pass through (often a silicone dome in the case of power LEDs).

This light is relatively monochromatic (its emission width in terms of wavelength is narrow, around a few tens of nanometers), and to be able to emit white light, a layer of phosphor makes it possible to convert this monochromatic light into broader spectrum light.

Depending on the manufacturer, the semiconductor’s power supply is made through gold wires (with a diameter of a few tens of microns) or electrical ones via methods through the semiconductor itself; a small metalized well that conducts the current.

By applying polarity to the terminals of the LED and injecting a current (controlled), the semiconductor will emit light (relatively monochromatic). Depending on the fabrication of the semiconductor and the different materials used, the color emitted will be different, ranging from UV (some LEDs emit around 275 nm) to near infrared (850 nm).

The main difference between white and colored LEDs is that white LEDs have an additional yellow-orange luminophore to reconstitute a relatively flat emission spectrum. For this result, the semiconductor emits blue (and monochromatic) light, which is partially absorbed by the phosphor and transformed by it into colors ranging from green to red. The superposition of this emission and the blue absorbed recreates the spectrum of white light or at least brings it closer to it.

The first white LEDs manufactured in the late 1990s used luminophores of very relative quality, and the method of deposition of these luminophores, coupled with this quality, often resulted in bluish spectra, where the unabsorbed blue portion was difficult to control and often too high (cold color temperature of the LEDs).

Another reason for this colorimetry is the lack of performance of the blue semiconductor chips of that time did not allow the use of more absorbent phosphors (to restore the other colors of the spectrum), which for many of them already existed derived from previous technologies. We should recall that Nichia, currently one of the leaders in the manufacture of LEDs, was above all a manufacturer of phosphors, particularly for the fluorescent tube market.

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