LEDs

Overview of LEDs

Light Emitting Diodes, or LEDs, are semiconductor diodes that harness the photons of light released when electrons lose energy and fall from a high energy state into a lower energy position (or “hole”) in the semiconductor’s atomic structure, a phenomenon known as “electroluminescence”.

Early versions of LEDs released only red light of limited brilliance but today, depending on the materials used for the semiconductor, they are manufactured to emit exceptionally brilliant light and are available in the non-visible (ultra-violent and infra-red) spectrum as well as the visible spectrum, where they come in a broad range of colours. Different semiconductor materials have different energy band gaps, hence the different wavelengths of light emitted during electron energy loss.

Increasingly, they are used in general domestic and commercial or industrial lighting systems as well as in automobile lighting systems, aviation lighting, and traffic lights; they are also increasingly used as indicator lamps in a huge array of different devices. Infrared LEDs are still widely used in remote control handsets for DVD and MP3 players as well as TV sets and the have enabled a new generation of ultra-brilliant video and text displays. Additionally, they are being taken up in a new generation of internet connection technology called “Light Fidelity” of LiFi, which may supplant WiFi.

They are considerable smaller than conventional incandescent light sources such as light bulbs (whether energy saving or not), and are often no more than a square millimetre or two in area. They are also considerably more durable than these conventional sources and consume vastly less energy. Currently, however, the initial investment for using them in lighting systems is much higher than for conventional light sources, although they last far longer and consume much less energy over their lifespans.

 

History of LEDs

Forerunners to the modern LED can be traced back to 1907, when the British scientist Henry Round discovered that a silicon carbide crystal would be induced to emit light through electroluminescence (i.e. releasing energy primarily from electrons in the form of photons instead of heat as they dropped from a higher energy hole to a lower energy hole). In 1955, the American radio engineer Rubin Braunstein reported infrared electroluminescent emissions from the semiconductor allow gallium arsenide (GaAs), yet it wasn’t until 1962 that the first patented LED came onto the world stage, when the Texas-based electrical engineers James Baird and Gary Pittman adapted the GaAs semiconductor material to include a p-n (positive-negative) junction. This became the world’s first patented infra-red LED.

In the same year, the American scientist Nick Holonyack invented the first LED to emit light in the visible range. But they remained exceedingly expensive and were not widely taken up commercially until 1968, when the Monsanto Company used gallium arsenide phosphate to mass-produce a much less costly and commercially viable LED, which began to be used in indicators on a huge scale (especially the seven-segment displays commonly found on calculators, digital clocks and watches).

LED lamps for lighting purposes began to hit the markets in 1994, when the Nichia Corporation’s Shuji Nakamura developed the first high-brilliance white LED. By 2012, the light manufacturer Osram commercially produced high-brilliance LEDs, using the semiconductor indium gallium nitride (InGaN) on a silicon substrate.

 

Technical aspects

Semiconductor materials can be treated (or “doped”) to become n-type semiconductors, which have a surfeit of electrons, or p-type semiconductors, which have a deficit. Both are used in LEDs in the form of a p-n junction: current will flow much more easily from the positive (anode) side of the junction to the negative (cathode) side than in the reverse direction. The p and n sides set up a differential voltage causing electrons to release their energy as light while they flow into low-energy holes.

Most LED indicators consume between 30 and 60 thousandths of a watt in electrical power. A major advantage of LED lamps used in lighting systems is that they demonstrate exceptionally high luminous efficacy (the relationship of luminous flux to power consumed, measured in lumens per Watt or lm/W). They have now not only matched, but also exceeded, the efficacy of conventional incandescent lights – a typical five-watt LED has a luminous efficacy of 18-22 lm/W, whereas a standard, 60-100 watt incandescent electrical light bulb emits just 15 lm/W.

The luminance of LED lighting is increased not by increasing current but by combining numerous LEDs in a single bulb (increasing current alone merely decreases the efficiency of an LED, a phenomenon known as “efficiency droop”).

Typically, LEDs have lifespans of between 25,000 and 100,000 hours (roughly, between 3 and 11 years). Although they are produced in a broad range of shapes and sizes, the colour of their plastic lenses is not generally responsible for the colour of light emitted – the lenses generally match the colour of the light emitted from LED itself.

 

Where LEDs are used in manufacturing – any famous instances?

In addition to the applications mentioned earlier (text and video displays, li-fi, aviation lighting, automotive lighting and general lighting), LED security systems are growing in popularity. Not only are infra-red LEDs increasingly being used in night vision devices, they can also be hooked up to security cameras and automated security floodlight systems.

 

How LEDs differs from other lights

Unlike other incandescent light sources, LEDs use semiconductors as the substrate within which electroluminescent reactions occur. They last considerably longer and are more efficient than conventional alternatives.