Lighting – Light Emitting Diodes [Part 1 – Theory]

Efficient lighting is achieved when most of the energy conversion occurs when electrons transition between high and low orbitals. Maximize electron energy and minimize molecular/atomic energy.

Despite the daunting physics behind the devices, the operation of the first forms of electrical lighting have been pretty simple. Dump a lot of energy into a source of matter (solid or gas) until it glows.

The innovation behind the light emitting diode [LED] is very different in this aspect, requiring only a fraction of the energy in comparison to incandescent and fluorescent light sources. This is observable in all aspects of technology, as LEDs are being incorporated into just about everything.

A very popular use for LEDs is in the detection of current within a circuit (connected in parallel, of course). This gives the user an extremely easy visual signal with minimal power consumption and user effort required (since multimeters are so Old School)!

LEDs are essentially electrical “gates” (technically called diodes) that allow electrons to only flow through if given a decent enough “push” (a couple AAA batteries will do just fine). When the electron passes through, it “churns” the gate which releases “energy” in the form of a particle of light.

That was a bad analogy; way too abstract. Let’s try that again.

The production of light within an LED occurs at the interface between a two-layer system. These layers play the roles of electron orbitals, similar to the gas-discharge system described previously. One layer acts as a “high orbital” state at a specific energy, while the other acts as the “low orbital” state.

With an applied current, the majority of the electrons progress, traversing high-to-low energy states. These electrons release light particles [photons] when they make the transition. Since the materials do not change, the color of light emitted is (relatively) constant. LEDs only give off a single color, which is determined by the energy difference between the LED’s two layers.

A rule of thumb; a higher energy differential is typically harder to manufacture. Blue LEDs were truly worth a Nobel prize, as seen within the past year.

Note: Most developed LEDs consist of >3 layers. While only one interface between two of these layers results in light production, additional layers play significant roles in optimizing efficiency. A layer can assist in an electron’s transition between metal and semiconductor, reducing device heating.  Other layers can decrease the amount of leaky current (kind of like duct tape around a bad hose).

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“White LEDs” are a misnomer to some extent, because white is a mixture of colors that a single LED cannot accomplish alone. There are two ways to make an LED white.

  1. Place three LEDs (red, green, and blue) in extremely close proximity to each other and turn them on. One can change the LED’s color by changing the current flow between each of the LEDs, allowing for a nice nightlight effect!
  2. Take a blue LED and give it a phosphor coating. A large portion of the blue light undergoes fluorescence in the phosphor and is re-emitted as green/yellow/orange/red light. This is the more popular method since phosphor coatings are typically cheaper to incorporate.

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In the next post, I will (attempt to) write about the fabrication process behind LED manufacturing. And I hope it’s quite the treat!

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