What is an LED?

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To understand the structure of the LED, it’s important to first understand the basics of the diode.

A diode is a two-lead semiconductor device that acts as a one-way gate to electric current flow. When a diode’s anode lead is made more positive in voltage than its cathode lead—a condition referred to as forward biasing—current is permitted to flow through the device. Diodes are commonly employed in rectifier and voltage regulating circuits.

A pn-junction diode is a semiconductor made up of two separate semiconductors, an n-type crystal on one side and p-type crystal on the other. The two conductors have two unique ways of passing an electric current—the p side does so with holes, the n side with electrons. The boundary between the two semiconductors is called the pn junction. When this pn junction is forward-biased, electrons in the n side are excited across the junction to the p side. On the p side, the electrons combine with electron holes and photons are emitted.

A light-emitting diode (LED) is a pn-junction diode designed to emit visible or infrared light (LEDs only give off a particular color photon—standard photon colors are red, yellow, green, and infrared.). Like pn-junction diodes, LEDs are current-dependent. To control the output intensity of an LED —and, of course, to emit light—the forward current is varied. For visible-light LEDs, maximum forward voltages are about 1.8 V, with typical operating currents from 1 to 3 mA. For infrared LEDs, maximum forward voltages at specific forward currents range from about 1.60 V at 20 mA to 2.0 V at 100 mA.

With its perennial potential as a superior conductor for lighting, telecommunications, lasers, and beyond, the LED has been the primary research focus of the lighting industry throughout the last 80 years. In the late 1920s, Russian physics technician Oleg Vladimirovich Losev was the first to record the light emission of silicon carbide diodes. The glowing diode (now known as the light-emitting diode) has well exceeded initial expectations, as it was first considered only a simple optical relay for telegraphic and telephone communications.

An accidental by-product of the race to develop better semiconductors, the infrared LED was discovered in 1961 when physicists James Baird and Gary Pittman at Texas Instruments were attempting to make an X-band Gallium Arsenide (GaAs) varactor diode. Hot on the heels of the TI discovery, in 1962 physicists Nick Holonyak and Robert Hall of GE, Marshall Nathan of IBM, and Robert Rediker of MIT simultaneously produced GaAsP (Gallium Arsenide Phosphide) on a GaAs substrate, the first visible-spectrum LED, effectively ushering in the modern era of LED technology.

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In 1962, the findings of the Holonyak group marked the beginning of the viable light-emitting pn-junction LED. The first commercial LED and its soon-to-be ubiquitous red glow entered the market shortly after. Unfortunately, at over $200 per LED, the cost made it less than widely accessible. However, the market was starting to change by the late 1960s as the potential of the LED became more and more obvious. Commercial mass production began in earnest with Monsanto, followed by Hewlett-Packard (Monsanto initially supplied GaAsP to HP until HP started growing GaAsP itself).

By the 1970s, LEDs were everywhere: as numeric displays for calculators, watches, and watch-calculators and as backlighting for telephone dials. The increase in applications kept pace with the increase in technology. Industry experts were able to improve the buffer layer between the GaAsP and the GaAs substrate, dramatically increasing brightness, and doping with optically active impurities dramatically increased efficiency.

Whereas other light sources have distinct fundamental restraints, the solid-state LED is limited only by the ingenuity of the present-day engineer. Offering what is inconceivable with conventional sources, new smart technology can control the spectral, spatial, temporal, and polarization properties of LEDs as well as the color temperature, proving the sky is not the limit with LED-based technologies. In fact, currently emerging technologies are expected to bring about tremendous benefits in the fields of lighting, automobiles, transportation, communication, imaging, agriculture, and medicine.

With incredible efficiency and an environmentally stable design, the LED is undeniably the lighting technology of the future. As such, the lighting industry is racing to develop an LED capable of producing illumination-grade white light. Enter the phosphor-converted (pc-)LED. Pc-LEDs emit high-energetic blue radiation and are coated with different red-shifting luminescent materials (e.g. phosphors). To obtain a white-light pc-LED, either a broadband yellow emitting (1-pc-LED) or a mixture of red and green phosphor materials (2-pc-LED) is used in addition to a blue LED die. The additive mixing of the initial blue light with the emission of different phosphors produces white light.

This research is not without its challenges. Most commercial LEDs use garnet materials doped with Ce3+ as the yellow broadband emitter due to its excellent thermal and chemical stability. But, it lacks emission in the red spectral range, limiting its application to cool white light. Although the search for a red-emitter that can meet the demanding design requirements of the LED has not yet fully materialized, researchers have discovered that (oxo)nitridosilicates used as host lattices for doping with rare-earth ions produces luminescent materials, covering the whole spectral range from blue to red. Unfortunately, meeting this high color-rendering index comes at the cost of lower luminous efficacy.

The challenge lies in improving the color rendition without comprising energy efficiency. Next-generation red phosphors like Sr[Mg3SiN4]:Eu2+ (SMS) and Sr[LiAl3N4]:Eu2+ (SLA) show the potential to have ideal  phosphor characteristics for use in pc-LEDs. These highly condensed nitrides show narrow-band emissions in the red-spectral region. Narrow emissions in this region are both within the sensitivity range of the human eye and are energy and thermally efficient.

Despite the advantages of LED energy efficiency, thermal management proves to be a distinct disadvantage when applying the technology. Heat is directly related to the lifetime of an LED, and LEDs produce a lot of heat at the semiconductor junction—LED lighting emits 25% visible light and 75% heat. For example, chip on board (COB) LEDs can create 50W of heat or more in a very small package. Since the rule of thumb is to allow four square inches of heat sinking per watt of power, space becomes a concern even when the LED itself is as small as 10mm square. In order to meet acceptable lifetime standards, various heat sinking schemes have been developed to dissipate at least some of the heat generated by the LED. On a bright note, LED lighting emits convective heat rather than radiant heat, and therefore, most of the LED-generated heat can be reused and removed by transporting it to another space.

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Since best practice dictates that LED development should concentrate on optimal design at the chip level to increase light extraction efficiency, thermal management of high-power LEDs (especially the COB due to its high heat flux in a small area) using alternative thermal operating systems (ATOS) will be an essential focus of future R&D. Since new research has already shown that an ATOS is a viable synergetic solution to current limitations, the wait most likely will not be long. In the meantime, the potential energy savings of LED technology far outweighs any setbacks due to the current limitations in thermal management, and researchers will without a doubt continue to pursue LED development to its absolute ends.

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