Light Emitting Diode / LED

The LIGHT EMITTING DIODE (LED) is a diode that emits light when a sufficiently large forward current is passed through it. Its construction differs from an ordinary rectifier diode: the n-region is doped more heavily than the p-region, and the p-n junction is encased inside a dome-shaped transparent case, which lets light escape uniformly in all directions while minimising internal reflections that would otherwise trap and waste light within the device. Metal electrodes attached to either side of the junction serve as the external electrical contacts; by convention, the longer of an LED's two legs is its positive terminal, the anode. LEDs with more than the usual two pins (3, 4, or 6-pin packages) are also available, allowing multiple colours to be produced from a single LED package, and surface-mounted LED displays are common for mounting directly onto PCBs.\n\nWorking (Fig. 16.16): when the LED is forward biased, electrons from the semiconductor's conduction band recombine with holes in the valence band near the junction, and each such recombination event releases its excess energy as a PHOTON -- since this energy corresponds to a well-defined value (approximately the material's band gap), the emitted light is essentially MONOCHROMATIC (a single colour). Because the junction region is very thin, a substantial fraction of these photons can escape the device rather than being reabsorbed, and the LED is fabricated so that recombination happens preferentially at the surface, maximising light output; the amount of light emitted is directly proportional to the forward current -- more current, more light. The LED's I-V characteristic (Fig. 16.17) resembles an ordinary forward diode's, but with a noticeably HIGHER cut-in (turn-on) voltage, typically in the range 1.5 to 3.5 V (versus roughly 0.6-0.7 V for silicon diodes); because the LED's current rating is only a few tens of milliamps, a suitably high series resistance must always be connected with it to limit current.\n\nThe wavelength (and hence colour) of light an LED emits is set by the semiconductor material used and, more precisely, by its band gap -- by varying the elements and their proportions in a compound semiconductor, different colours can be produced: aluminium gallium arsenide (AlGaAs) emits infrared radiation; gallium arsenic phosphide (GaAsP) produces red or yellow light; aluminium gallium phosphide (AlGaP) produces red or green light; and zinc selenide (ZnSe) produces blue light. (A blue LED, which enabled the later development of white-light LEDs via wavelength conversion -- typically exciting a yellow phosphor with blue LED light, whose combined blue-plus-yellow emission appears white -- was itself a landmark invention recognised with a Nobel Prize in Physics.) Advantages of LEDs include being a solid-state light source with no filament, tube, or bulb to break; high energy efficiency (substantial light output per watt of input power); very long operating lifetimes; almost no warm-up delay (light output begins within nanoseconds); excellent, non-fading colour rendering, ideal for …