Light-Emitting Diodes: A Primer
Semiconductors called light-emitting diodes (LEDs) transform electrical energy into light energy. The semiconductor material and composition determine the colour of the output light, with LEDs often being categorised into three wavelengths: ultraviolet, visible, and infrared.
The available commercially available LEDs with single-element output power of at least 5 mW have a wavelength range of 275 to 950 nm. Regardless of the manufacturer, a particular semiconductor material family is used for each wavelength range. An overview of LED functioning and a quick glance at the sector are provided in this article. There will also be a discussion of different LED kinds, the appropriate wavelengths, the materials utilised in their construction, and some uses for the particular lights.
UV LEDs (ultraviolet LEDs): 240 to 360 nm
Particularly for water disinfection, medical/biomedical applications, and industrial curing, UV LEDs are employed. At wavelengths as short as 280 nm, power output levels greater than 100 mW have been accomplished. Gallium nitride/aluminum gallium nitride (GaN/AlGaN) with wavelengths of 360 nm or longer is the material most frequently utilised for UV LEDs. Shorter wavelengths make use of exclusive materials. Shorter wavelengths are produced by only a few providers, and the costs for these LEDs are still quite high when compared to the rest of the LED product offers, even if the market for wavelengths 360 nm and longer is stabilising due to reduced pricing and a large supply.
Green LEDs range from near-UV to 530 nm
Indium gallium nitride (InGaN) is the material used for the goods in this wavelength range. While it is technically feasible to produce an LED with a wavelength of any value between 395 and 530 nm, the majority of major suppliers focus on generating blue LEDs (450 to 475 nm) for phosphor-based white illumination and green LEDs in the 520–530 nm range for traffic signal green lighting. Most people consider the technology behind these LEDs to be advanced. Over the past few years, improvements in optical efficiency have slowed or ceased.
LEDs ranging from yellow-green to red: 565 to 645 nm
The semiconductor substance utilised for this wavelength range is aluminium indium gallium phosphide (AlInGaP). It is mostly produced in traffic signal yellow (590 nm) and red (625 nm) wavelengths. Although they are less common, the lime-green (or yellowish-green 565 nm) and orange (605 nm) are also offered in this technology.
It's noteworthy to notice that the pure green (555 nm) emitter is not a feature of either the InGaN or AlInGaP technologies. There are older, less effective technologies in this area of pure green, but they are not thought to be efficient or brilliant. This is mostly caused by a lack of financing for the development of alternative material technologies for this wavelength range as well as a lack of commercial interest or demand.
660 to 900 nm: deep red to near-infrared (IRLEDs)
The construction of devices in this area can take many different forms, but they always use aluminium gallium arsenide (AlGaAs) or gallium arsenide (GaAs) elements. Numerous medical uses (at 660–680 nm) as well as infrared remote controls and night-vision lights are among the applications.
LED operation theory
An electrical voltage that is sufficient for the electrons to move across the depletion region and combine with a hole on the other side to create an electron-hole pair must be applied in order for LEDs, which are semiconductor diodes, to emit light when an electrical current is applied in the forward direction of the device. This causes the electron to emit a photon as it releases its energy in the form of light.
The wavelength of light emitted depends on the semiconductor's bandgap. Higher-bandgap materials emit shorter wavelengths because shorter wavelengths have more energy. greater voltages are also necessary for conduction in materials with a greater bandgap. While near-IR LEDs have a forward voltage of 1.5 to 2.0 V, short-wavelength UV-blue LEDs have a forward voltage of 3.5 V.
Availability and efficiency factors for wavelengths
Market potential, consumer demand, and industry-standard wavelengths are the main determinants of whether a certain wavelength is commercially viable or not. This is most noticeable in the 420–460 nm, 480–520 nm, and 680–800 nm wavelength ranges. There are no high-volume manufacturers producing LED devices for these wavelength ranges since there are no high-volume uses for them. Nevertheless, it is feasible to locate small- or medium-sized vendors that provide goods to fill these specific wavelengths on a bespoke basis.
The wavelength region where each material technology is most effective may be found pretty nearly in the centre of each range. Efficiency diminishes as the semiconductor's doping level rises or falls below the ideal level. For this reason, a blue LED produces far more light than a green or near-UV LED, amber produces more light than a yellow-green LED, and near-IR produces more light than 660 nm. Designing for the middle of the spectrum rather than the edges is always a better option. Additionally, it is simpler to get goods that do not straddle the frontiers of material technology.
Supplying LEDs with current and voltage
LEDs are diodes and must be operated in a current mode even though they are semiconductors and require a minimum voltage to function. When using LEDs in DC mode, there are two primary methods: The use of a current-limiting resistor is the simplest and most popular. The considerable heat and power dissipation in the resistor is a drawback of this technology. The supply voltage needs to be substantially higher than the forward voltage of the LED for the current to remain steady across temperature changes and from one device to another.
Commercial off-the-shelf LED drivers are offered by a variety of suppliers. For brightness control, they typically function utilising pulse width modulation principles.
A distinct set of issues arises when pulsing LEDs in high-current and/or high-voltage mode for arrays connected in series and parallel. It is not practicable for a beginning designer to create a current-controlled pulse drive that can provide 5 A and 20 V. A few companies produce specialised tools for LEDs that pulse.
LEDs in applications that people can see
Exact colour matters significantly more in situations where LEDs are directly viewed or utilised as luminators than precise output in lumens or candela. The brain makes excellent adjustments for any variations in light intensity while the human eye is comparatively indifferent to them. The average person viewing an LED video screen on a building, for instance, won't notice a 20% reduction in intensity as parts of the screen are viewed at 10° to 20° off-axis compared to the portion directly on-axis because this is a gradual change that is not perceived as it moves towards the edge of vision. In contrast, the human eye will notice a colour variation and find it bothersome if an area's LEDs have a 10 nm wavelength difference from those in other areas.
Most white LEDs in use today are created by infusing a longer wavelength visible phosphor with a blue LED. The spectral resemblance to sunshine is measured by the colour rendering index (CRI). The majority of LEDs used in general lighting nowadays have a CRI better than 80, with 100 being regarded as being equivalent to sunshine. White LEDs are becoming the most sought-after product for the majority of lighting applications due to CRI advancements and improved optical efficiency.
Benefits and uses of LED
In comparison to filtered lights, LEDs have several benefits for monochromatic applications since their wavelength spectra are more precisely specified. The energy savings from employing a filtered incandescent bulb for general illumination applications can potentially be 100 times higher. Applications like traffic signals and architectural lights benefit greatly from this. A tiny solar panel can readily power low-power portable highway LED signs in place of a big generator, which is a clear benefit.
In general, LEDs are less expensive, more dependable, and may be powered by cheaper electronics than lasers. LEDs are now classified separately by both the U.S. and the European Union. Fortunately, unlike lasers and laser diodes, LEDs do not come with the same eye safety issues or warnings. On the other hand, it is impossible to create optically dense, very tiny, and highly collimated spots with LEDs. A laser is nearly always needed in applications that call for exceptionally high power density in a compact region.
Today, LEDs are utilised in a wide range of sectors and applications (Table 1). These devices are extremely economical and appealing to both consumer and industrial markets thanks to their great dependability, high efficiency, and reduced total system cost compared to lasers and lamps. Each unique LED technology and/or colour has been created to meet a particular use's needs.
