Light Emitting Diodes (LED)

Light Emitting Diodes (LED)

Introduction

Light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode that emits light when activated. When a suitable current is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons.   This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LEDs are typically small (less than 1 mm²) and integrated optical components may be used to shape the radiation pattern. Unlike a laser, the color of light emitted from an LED is neither coherent nor monochromatic, but the spectrum is narrow with respect to human vision, and for most purposes the light from a simple diode element can be regarded as functionally monochromatic

  LEDs are like tiny light bulbs. However, LEDs require a lot less power to light up by comparison. They’re also more energy efficient, so they don’t tend to get hot like conventional light bulbs do   this makes them ideal for mobile devices and other low-power applications The LED is a light source which uses semiconductors and electroluminescence to create light.  LED   uses a small semiconductor crystal with reflectors and other parts to make the light brighter and focused into a single point. Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get especially hot. They are illuminated solely by the movement of electrons in a semiconductor material, and they last just as long as a standard transistor. The lifespan of an LED surpasses the short life of an incandescent bulb by thousands of hours. Tiny LEDs are already replacing the tubes that light up LCD HDTVs to make dramatically thinner televisions.

History

LEDs appeared as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still frequently used as transmitting elements in remote-control circuits, such as those in remote controls for a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.

Working

The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.

The wavelength of the light emitted, and thus its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes usually recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

If you connect an LED directly to a current source it will try to dissipate as much power as it’s allowed to draw, and, like the tragic heroes of olde, it will destroy itself. That’s why it’s important to limit the amount of current flowing across the LED.. Resistors limit the flow of electrons in the circuit and protect the LED from trying to draw too much current. 

LEDs create light by electroluminescence in a semiconductor material. Electroluminescence is the phenomenon of a material emitting light when electric current or an electric field is passed through it - this happens when electrons are sent through the material and fill electron holes. An electron hole exists where an atom lacks electrons (negatively charged) and therefore has a positive charge. Semiconductor materials like germanium or silicon can be "doped" to create and control the number of electron holes. Doping is the adding of other elements to the semiconductor material to change its properties. By doping a semiconductor you can make two separate types of semiconductors in the same crystal. The boundary between the two types is called a p-n junction. The junction only allows current to pass through it one way, this is why they are used as diodes. LEDs are made using p-n junctions. As electrons pass through one crystal to the other they fill electron holes. They emit photons (light).

Applications

Early LEDs were often used as indicator lamps for electronic devices, replacing small incandescent bulbs. They were soon packaged into numeric readouts in the form of seven-segment displays and were commonly seen in digital clocks. Recent developments have produced LEDs suitable for environmental and task lighting. LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology. . Light-emitting diodes are used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper and medical devices. They form numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo television screen or illuminate a traffic light.Oter uses incuude :Indicator lights ; LCD panel backlighting Specialized white LEDs are used in flat-panel computer displays ;Fiber optic data transmission Ease of modulation allows wide communications bandwidth with minimal noise, resulting in high speed and accuracy ; Remote control Most home-entertainment "remotes" use IREDs to transmit data to the main unit; Optoisolator Stages in an electronic system can be connected together without unwanted interaction.

,LEDs serve as the foundation of networked lighting systems that also provide information to other systems connected to an internal building management network. For example, the heating, ventilation, and air conditioning systems could be alerted that certain parts of the building are empty, and could adjust the temperature or shut off the air conditioning entirely.

Energy savings

LEDs use about 85% less electricity than incandescent bulbs and as much as 50% less than fluorescents. The amount saved with fluorescents will vary depending on whether you’re using a fluorescent tube or a compact fluorescent bulb (CFL). The efficiency of a light bulb is measured in lumens per watt and currently LED technology is providing higher lumens per watt than fluorescent, but is not practical for every application a fluorescent source is being used.

One of the main problems with incandescent bulbs is they emit a great deal of energy as heat (that’s why you can burn yourself if you touch a lit incandescent bulb), and this heat signifies that some energy is being wasted instead of converted to light. On the other hand, if you touch an LED light it is typically cool to the touch and will likely not notice any heat at all. This is because more electricity is being converted to light — about 85% more.

A few of the major advantages of LEDs over CFLs are they don’t produce any UV rays, last 5 times longer, save 20% in energy costs and are lead and mercury free.

By design LEDs can last as much as 50 times longer than other bulbs and have lifetimes ranging from 30,000 to 100,000 hours or more at constant operation. Depending on how many hours a day your facility is lit, this can equal a lifespan of anywhere from 6 to 30 years. In comparison, incandescent bulbs only last an average of 1000 to 5000 hours, CFLs last 8,000 to 10,000 hours, and fluorescent tubes have lifetimes of 20,000 to 50,000 hours.

Part of the reason LEDs last so long is because they are durable (no glass components) and do not have a filament (like incandescent bulbs) that can break our burn out. Their illumination comes exclusively from the movement of electrons in a semiconductor material.

Lighting accounts for about 20 percent of the total energy usage worldwide, approximately 1,944 terawatt hours. Because lighting is relatively simple to upgrade, the trend in recent years is to switch to LEDs. Doing so greatly improves energy efficiency; mitigating global warming’s impact and reducing energy dependence on other countries.

Globally, LEDs make up less than 10 percent of lighting systems, according to the U.S. Department of Energy Solid-State Lighting R&D Plan, published in June 2016. But the DOE forecasts LEDs to become the predominant source of illumination—for indoor and outdoor spaces—over the next decade. The DOE’s “Energy Savings Forecast of Solid-State Lighting in General Illumination Applications,” released in September, predicts that by 2035, LED lamps and luminaires will be used in 86 percent of all lighting products in the United States, compared with 6 percent in 2015. That translates into an annual savings of 1,495 terawatt hours over traditional lighting systems, nearly the total annual energy consumed by 45 million U.S. homes today. This adds up to nearly $52 billion in energy costs savings.

Because LEDs are semiconductor devices, integrating additional electronics in the bulbs, such as occupancy and daylight sensors connected to a network interface, provides the bulbs with new functions. This design enables, for example, the automatic dimming of lights when there’s sufficient natural light in the room, and it senses the presence of people in a room and adjusts overhead lighting accordingly. These capabilities not only provide the optimal amount of lighting throughout a building but also save energy and money.

LEDs also help lower electricity costs in exterior lighting systems. Those used in parking lots, garages, and walkways can be configured to automatically dim or turn lights off when sensors detect an unoccupied area, and much faster than today’s systems. This type of automation is an especially attractive cost-saving option for cities. About half of a city’s monthly electricity budget is devoted to keeping the streetlights on.

Other Benefits  

LEDs also are changing the way architects and interior designers use lighting. Because the bulbs are smaller and weigh less than traditional ones, LED products allow for greater flexibility and creativity in lighting venues such as concert halls, museums, and retail stores, where aesthetics are especially important, while reducing energy usage.

LEDs in malls can motivate people to linger in a store longer and therefore boost sales. The ability to adjust from warmer to cooler shades of white, referred to as color tuning—along with precise light distribution—allows the retailer to create an inviting space that could appeal to the shopper’s sense of comfort, safety, and familiarity. In the home, LED lights could automatically mimic a bright, sunny day even if it is gloomy out. Such lighting systems will allow us to adjust our environments in ways that older technologies never could.

High Upfront Cost

Although LED prices have dropped dramatically, and continue to do so as technology advances, they typically cost more than other lighting systems. However, their high level of efficiency means you can recoup your upfront costs in a relatively short period of time. Besides requiring less electricity, LED’s long lifespan also saves you money on maintenance and replacement bulbs,  .

 Advantages:

The U.S. Department of Energy sees LEDs as the lighting source with the greatest potential for the future. They predict that, with widespread adoption of LEDs, by 2025 the country will:

·         Lower electricity demands for lighting by 62%.

·         Reduce carbon emissions by 258 million metric tons.

·         Diminish amount of materials in landfills.

·         Prevent construction of 133 new power plants.

·         Save $280 billion.

Although LEDs aren’t practical for every application (yet), they are certainly the lighting choice of the future and offer huge benefits, including energy savings, for your facility and the country as a whole.

LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. They are also significantly more energy efficient and, arguably, have fewer environmental concerns linked to their disposal.

Benefits of LEDs and IREDs, compared with incandescent and fluorescent illuminating devices, include: Low power requirement,  Most types can be operated with battery power supplies; High efficiency Most of the power supplied to an LED or IRED is converted into radiation in the desired form, with minimal heat production; Long life When properly installed, an LED or IRED can function for decades.

The evolution of LEDs as a source of white light has progressed dramatically in the last two decades, especially in terms of efficacy, or how well a light source produces visible light, measured in output lumens/input watts. The increase in efficacy has been achieved primarily through advances in LED chip technology, and is helping to drive LED adoption. LED products are also more affordable than incandescent bulbs over their life cycle, because their power requirements are lower and the bulbs themselves can last more than 10 years, compared with today’s incandescent bulbs they are replacing, which often last up to a year or so. Advntgaes of LEDs include:

-Energy efficient source of light for short distances and small areas. The typical LED requires only 30-60 milliwatts to operate
-Durable and shockproof unlike glass bulb lamp types
-Directional nature is useful for some applications like reducing stray light pollution on streetlights

Disadvantages:

-May be unreliable in outside applications with great variations in summer/winter temperatures, more work is being done now to solve this problem
-Semiconductors are sensitive to being damaged by heat, so large heat sinks must be employed to keep powerful arrays cool, sometimes a fan is required. This adds to cost and a fan greatly reduces the energy efficient advantage of LEDs, it is also prone to failure which leads to unit failure
-Circuit board solder and thin copper connections crack when flexed and cause sections of arrays to go out
-Rare earth metals used in LEDs are subject to price control monopolies by certain nations
-Reduced lumen output over time

Efficiency

Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts (mW) of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt. These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for greater heat dissipation from the LED die.

One of the key advantages of LED-based lighting sources is high luminous efficacy. White LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with luminous efficacy of 18–22 lumens per watt (lm/W). For comparison, a conventional incandescent light bulb of 60–100 watts emits around 15 lm/W, and standard fluorescent lights emit up to 100 lm/W.

As of 2012, Philips had achieved the following efficacies for each color. The efficiency values show the physics – light power out per electrical power in. The lumen-per-watt efficacy value includes characteristics of the human eye and is derived using the luminosity function.

	Color	Wavelength range (nm)	Typical efficiency coefficient	Typical efficacy (lm/W)
	Red	620 < λ < 645	0.39	72
	Red-orange	610 < λ < 620	0.29	98
	Green	520 < λ < 550	0.15	93
	Cyan	490 < λ < 520	0.26	75
	Blue	460 < λ < 490	0.35	37

Red and Infrared LEDs are made with gallium arsenide
Bright Blue is made with GaN -gallium nitride
White LEDs are made with yttrium aluminum garnet

There are also orange, green, blue, violet, purple, ultraviolet LED

In September 2003, a new type of blue LED was demonstrated by Cree. This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescent. In 2006, they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. Nichia Corporation has developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA. Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100 lm/W at their full power of 10W, and up to 160 lm/W at around 2 W input power. In 2012, Cree announced a white LED giving 254 lm/W, and 303 lm/W in March 2014. Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.

These efficiencies are for the light-emitting diode only, held at low temperature in a lab. Since LEDs installed in real fixtures operate at higher temperature and with driver losses, real-world efficiencies are much lower. United States Department of Energy (DOE) testing of commercial LED lamps designed to replace incandescent lamps or CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17 lm/W to 79 lm/W).