Light-Emitting Diodes (LED)

Summary

Light-emitting diodes (LEDs) are diodes, semiconductor devices that pass current easily in only one direction, that emit light when current is passing through them in the proper direction. LEDs are small and are easier to install in limited spaces or where small light sources are preferred, such as indicator lights in devices. LEDs are also generally much more efficient at producing visible light than other light sources. As solid-state devices, when used properly, LEDs also have very few failure modes and have longer operational lives than many other light sources. For these reasons, LEDs are gaining popularity as light sources in many applications, despite their higher cost compared with other more traditional light sources.

Definition and Basic Principles

Diodes act as one-way valves for electrical current. Current flows through a diode easily in one direction, and the ideal diode blocks current flow in the other direction. The very name diode comes from the Greek meaning "two pathways." The diode-like behavior comes from joining two types of semiconductors, one that conducts electricity using electrons (n-type semiconductor) and one that conducts electrons using holes, or the lack of electrons (p-type semiconductor). The electrons will try to fill the holes, but applying voltage in the proper direction ensures a constant supply of holes and electrons to conduct electricity through the diode. The electrons and holes have different energies, so when the electrons combine with holes, they release energy. For most diodes this energy heats the diode. However, by adjusting the types and properties of the semiconductors making up the diodes, the energy difference between holes and electrons can be made larger or smaller. If the energy difference corresponds to the energy of a photon of light, then the energy is given off in the form of light. This is the basis of how LEDs work.

LEDs are not 100 percent efficient, and some energy is lost in current passing through the device, but the majority of energy consumed by LEDs goes into the production of light. The color of light is determined by the semiconductors making up the device, so LEDs can be fabricated to make light only in the range of wavelengths desired. This makes LEDs among the most energy-efficient sources of light.

Background and History

In 1907, H. J. Round reported that light could be emitted by passing current through a crystal rectifier junction under the right circumstances. This was the ancestor of the modern LED, though the term "diode" had not yet been invented. Though research continued on these crystal lamps, as they were called, they were seen as impractical alternatives to incandescent and other far less expensive means of producing light. By 1955, Rubin Braunstein, working at RCA, had shown that certain semiconductor junctions produced infrared light when current passed through them. Scientists Robert Biard and Gary Pittman, however, managed to produce a usable infrared LED, receiving a patent for their device. Nick Holonyak, Jr., a scientist at General Electric, then created a red LED—the viable and useful visual spectrum LED—in 1961. Though these early LEDs were usable, they were far too expensive for widespread adoption. By the 1970s, Fairchild Semiconductor had developed inexpensive red LEDs. These LEDs were soon incorporated into seven-segment numeric indicators for calculators produced by Hewlett Packard and Texas Instruments. Red LEDs were also used in digital watch displays and as red indicator lights on various pieces of equipment.

Early LEDs were limited in brightness, and only the red ones could be fabricated inexpensively. Eventually, other color LEDs and LEDs capable of higher light output were developed. Developing a blue LED proved to be particularly difficult, and for decades this prevented scientists from creating the white LED lights necessary for purposes such as normal indoor lighting. In the late 1980s and early 1990s, scientists at last succeeded in creating and refining a blue LED; three of these researchers, Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura, were granted the 2014 Nobel Prize in Physics for their efforts. As the capabilities of LEDs expanded, they began to see more uses. By the early twenty-first century, LEDs began to compete with other forms of artificial lighting based on their energy efficiency.

How It Works

An LED is a specific type of solid-state diode, but it still retains the other properties typical of diodes. Solid-state diodes are formed at the junction of two semiconductors of different properties. Semiconductors are materials that are inherently neither good conductors nor good insulators. The electrical properties of the materials making up semiconductors can be altered by the addition of impurities into the crystal structure of the material as it is fabricated. Adding impurities to semiconductors to achieve the proper electrical nature is called "doping." If the added impurity has one more electron in its outermost electron shell compared with the semiconductor material, then extra electrons are available to conduct electricity. This is a negative doped, or n-type, semiconductor. However, if the impurity has one fewer electron in its outermost electron shell compared with the semiconductor material, then there are too few electrons in the crystal structure, and an electron can move from atom to atom to fill the void. This results in a missing electron moving from place to place and acts like a positive charge moving through the semiconductor. Engineers call this missing electron a "hole," and semiconductors in which holes dominate are called "positive doped," or p-type, semiconductors.

The P-N Junction. To make a diode, a device is fabricated in which a p-type semiconductor is placed in contact with an n-type semiconductor. The shared boundary between the two types of semiconductors is called a "p-n junction." In the vicinity of the junction, the extra electrons in the n-type region combine with the holes of the p-type region. This results in the removal of charge carriers in the vicinity of the p-n junction and the area of few charge carriers is called the depletion region.

When a voltage is applied across the p-n junction, with the p-type region having the higher voltage, then electrons are pulled from the n-type region and holes are pulled from the p-type region into the depletion region. Additionally, electrons are pulled into the cathode (the exterior terminal connecting to the n-type region) replenishing the supply of electrons in the n-type region, and electrons are pulled from the anode (the exterior terminal connecting to the p-type region) replenishing the holes in the p-type region. This is the forward-bias orientation of the diode, and current flows through the diode when voltage is applied in this manner. However, when voltage is applied in the reverse direction, electrons are pulled from the n-type region and into the p-type region, resulting in a larger depletion region and fewer available charge carriers. Electric current does not flow through the diode in this reverse-bias orientation.

Electroluminescence. Electrons and holes have different energy levels. When the electrons and holes combine in the depletion region, therefore, they release energy. For most diodes, the energy difference between the p-type holes and the n-type electrons is fairly small, so the energy released is correspondingly small. However, if the energy difference is sufficiently large, then when an electron and hole combine, the amount of energy released is the same as that of a photon of light, and the energy is released in the form of light. This is called "electroluminescence." Different wavelengths or colors of light have different energies, with infrared light having less energy than visual light, and red light having less energy than other forms of visual light. Blue light has more energy than other forms of visual light. The color of light emitted by the recombination of electrons and holes is determined by the energy difference between the electrons and holes. Different semiconductors have different energies of electrons and holes, so p-n junctions made of different kinds of semiconductors with different doping result in different colors of light emitted by the LED.

Efficiency. Most light sources emit light over a wide range of wavelengths, often including both visual and nonvisual light as well as heat. Therefore, only a portion of the energy used goes into the form of light desired. For an idealized LED, all of the light goes into one color of light, and that color is determined by the composition of the semiconductors making the p-n junction. For real LEDs, not all of the light makes it out of the material. Some of it is internally reflected and absorbed. Furthermore, there is some electrical resistance to the device, so there is some energy lost in heat in the LED—but nowhere near as much as with many other light sources. This makes LEDs very efficient as light sources. However, LED efficiency is temperature dependent, and they are most efficient at lower temperatures. High temperatures tend to reduce LED efficiency and shorten the lifetime of the devices.

Applications and Products

LEDs produce light, like any other light source, and they can be used in applications where other light sources would have been used. LEDs have certain properties, however, that sometimes make their use preferable to other artificial light sources.

Indicator Lights. Among the first widespread commercial use of LEDs for public consumption was as indicator lights. The early red LEDs were used as small lights on instruments in place of small incandescent lights. The LEDs were smaller and less likely to burn out. LEDs are still used in a similar way, though not with only the red LEDs. They are used as the indicator lights in automobile dashboards and in aircraft instrument panels.

Another early widespread commercial use of LEDs was the seven-segment numeric displays used to show digits in calculators and timepieces. However, LEDs require electrical current to operate, and calculators and watches would rather quickly discharge the batteries of these devices. Often the display on the watches was visible only when a button was pressed to light up the display. However, the advent of liquid crystal displays (LCDs) has rendered these uses mostly obsolete since they require far less energy to operate, and LEDs are needed to light the display at night only.

Replacements for Incandescent Lights. Red LEDs became bright enough to be used as brake lights in automobiles. Red, green, and yellow LEDs are sometimes used for traffic lights and for runway lights at airports. LEDs are also used in Christmas-decoration lighting. They are also used in message boards and signs. In addition, LEDs are used for backlighting LCD screens on televisions, computers, and mobile phones. Colored LEDs were also frequently used in decorative or accent lighting, such as lighting in aquariums to accentuate the colors of coral or fish.

For many years, the biggest obstacle to replacing incandescent lights with LEDs for room lighting or building lighting was that they produced light of only one color. One way to overcome this is to use multiple-colored LEDs to simulate the broad spectrum of light produced by incandescent lights or fluorescent lights. However, arrays of LEDs produce a set of discrete colors of light rather than all colors of the rainbow, thus slightly distorting colors of objects illuminated by the LED arrays. The other main strategy for producing white light from LEDs is to include a phosphorescent coating in the casing around the LED. This coating provides the different colors of light that mimics the light of fluorescent bulbs; however, initially, such a strategy removed much of the efficiency of LEDs. Continued research overcame these difficulties, and by the middle of the 2010s, white LEDs started to become practical for area lighting, until high-efficiency white LEDs became common in flashlights, light bulbs, and even automobile headlights.

In addition to being energy-efficient, LEDs have many properties that make them attractive replacements for incandescent or fluorescent lights. LEDs typically have no breakable parts and being solid-state devices are very durable and have low susceptibility to vibrational damage.

Nonvisual Uses for LEDs. Infrared LEDs are often used as door sensors or for communication by remote controls for electronic devices. They can also be used in fiber optics. The rapid switching capabilities of LEDs make them well suited for high-speed communication purposes. Ultraviolet LEDs are being investigated as replacements for black lights for the purposes of sterilization, since many bacteria are killed by ultraviolet light.

In the medical field, LEDs can be used in phototherapy to heal wounds by promoting blood circulation and anti-inflammatory processes. It can increase blood flow to the brain to improve cognitive ability. It is also beneficial to mitochondria, the powerhouse of the cell that produces an energy-carrying chemical called adenosine triphosphate (ATP). It can treat certain skin conditions, including acne vulgaris and psoriasis. Photodynamic therapy (PDT) helps in the treatment of cancerous skin lesions.

Careers and Course Work

LEDs are used in many industries, not just in electronics, which means that there are many different degrees and course-work pathways to working with LEDs.

The development of new types of LEDs requires detailed understanding of semiconductor physics, chemistry, and materials science. Typically, such research requires an advanced degree in physics, materials science, or electrical engineering. Such degrees require courses in physics, mathematics, chemistry, and electronics. The different degrees will have different proportions of those courses.

The utilization of LEDs in circuits, however, requires a quite different background. Technicians and assembly workers need only basic electronics and circuits courses to incorporate the LEDs into circuits or devices. Other courses include LEDs and semiconductor lasers, biophysics, and nanophotonics and detectors.

Aspirants can work as biophotonics engineers, display technologists, or optical engineers. Lighting technicians and lighting engineers also work with LEDs in new applications. Such careers could require bachelor's degrees in their field. New LED lamps are being developed and LEDs are seen as a possible energy-efficient alternative to other types of lighting. They also have long operational lives, so there is continual development to include LEDs in any type of application where light sources of any sort are used.

Social Context and Future Prospects

At first, LEDs were a niche field, with limited uses. However, as LEDs with greater capabilities and different colors of emitted light were produced, uses began to grow. LEDs have evolved past the point of simply being indicator lights or alphanumeric displays. Developments in semiconductor manufacturing have driven down the cost of many semiconductor devices, including LEDs. The reducing cost combined with the energy efficiency of LEDs has led these devices to become more prominent, particularly where colored lights are desired. Research continues to produce newer LEDs with different colors, different power requirements, and different intensities. Newer techniques are being developed to produce white light using LEDs. These technological developments will make LEDs even more practical replacements for current light sources, despite their higher initial up-front costs. LED bulbs help to reduce greenhouse gas emissions as they consume less power.

Research continues on LEDs to make them more commercially and aesthetically viable as alternatives to more traditional light sources. However, research is also continuing on other alternative light sources. The highest-efficiency fluorescent lights have similar efficiencies to standard LEDs, but they cost less and are able to produce pleasing white light that LEDs do not yet produce. LEDs will continue to play an increasing role in their current uses, but it is unclear if they will eventually become wide-scale replacements for incandescent or fluorescent lights.

According to the US Department of Energy (DOE), the increased demand for LEDs in 2019 was due to its energy efficiency, reduced prices, and strict government regulations.

In 2020, LED sales declined globally due to the COVID-19 pandemic, however, its demand from the healthcare sector remained intact. A study found that ultraviolet LEDs could kill the human coronavirus, and their further effectiveness was under research. In 2021, the British government imposed a ban on the sale of halogen and fluorescent bulbs to tackle climate change.

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