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From the Q&A

With TJ Byers


All About LEDs

Question:

Can you refer me to a source on how LEDs are produced, the colors, and practical aspects of how they can be incorporated in new or existing circuits — including LED types, requirements, and precautions?

— Paul


Answer:

 An LED is a specialized diode — one that has a conduction band that’s much higher than a silicon or Schottky rectifier diode. When a forward diode goes into conduction, the electrons are excited to a higher energy level. When the electron returns to its former energy level, it emits a photon. In an ordinary diode, the energy gap is small and most of the photon energy is emitted as infrared radiation — where it’s dissipated as heat.

FIGURE 1.


LEDs are specially constructed (Figure 1) to release a large number of photons, from infrared to ultraviolet. The color of the light is dependent on the energy level of the photon — which is directly related to the band gap. The more energy it takes to start conduction through the diode junction, the higher the energy of the photon and the shorter the wavelength. (Longer wavelengths, typically 700 nm and longer, emit infrared light; shorter wavelengths of 400 nm and shorter emit ultraviolet light.) The band gap (and resulting wavelength) is determined by the semiconductor material used to fabricate the diode junction (Table 1).

Wavelength (nm) Color Forward Voltage LED Material
385 Ultraviolet 3.8 GaN, InGaN, or SiC
395 Ultraviolet 3.8 GaN, InGaN, or SiC
405 Violet 3.8 GaN, InGaN, or SiC
430 Blue 3.8 GaN, InGaN, or SiC
450 Blue 3.8 GaN, InGaN, or SiC
470 Blue 3.8 GaN, InGaN, or SiC
490 Blue-Green 3.6 GaN, InGaN, or SiC
505 Blue-Green 3.6 GaN, InGaN, or SiC
525 Green 3.6 GaN, InGaN, or SiC
555 Green 2.2 GaP
565 Green-Yellow 2.2 GaP
570 Yellow-Green 2.2 GaP
585 Yellow 2.2 InGaAlP or GaP
590 Yellow 2.2 InGaAlP or GaP
592 Amber-Yellow 2.2 InGaAlP or GaP
600 Yellow 2.2 InGaAlP or GaP
612 Orange 2.0 InGaAlP or GaP
625 Red-Orange 2.0 InGaAlP or GaP
630 Red 2.0 InGaAlP or GaP
645 Red 2.0 InGaAlP or GaP
660 Red 1.9 AlGaAs
670 Infrared 1.8 AlGaAs
680 Infrared 1.8 AlGaAs
700 Infrared 1.8 AlGaAs
       
In = Indium Ga = Gallium Al = Aluminum As = Arsenic
P = Phosphide Si = Silicon C = Carbide N = Nitride

TABLE 1. Typical LED Construction.


Because LEDs are diodes, they are polarity sensitive, as shown in Figure 2. In the forward direction they conduct, in the reverse direction they block current flow. The amount of current going through the LED determines its brightness. More current, the brighter the LED.

FIGURE 2.


The current has to be limited by a resistor, otherwise the LED overheats and literally melts. LEDs are typically biased with 20 mA of forward current, which makes the limiting resistance equal to R = (Vsource - Vf) / LEDcurrent, where Vsource is the battery voltage and Vf is the forward voltage of the LED (Table 1). For further information on LEDs, check out The LED Light website at www.theledlight.com/technical.html


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