The facts of led light
In the field of electronics, an LED refers to a semiconductor device that emits infrared or visible light when supplied with an electric current. The application of LED displays in consumer electronic devices began in 1968, a year when Hewlett-Packard (HP) introduced the world's first LED display.
Visible LED lights are applied in a wide range of electronic devices: they function as indicator lamps, car brake lights, and are also used for alphanumeric displays-even extending to full-color posters on billboards and signs. As for infrared LEDs, they are employed in autofocus cameras and TV remote controls, and also serve as light sources in fiber-optic telecommunication systems.
The once-common yet now obsolete incandescent light bulb produced light via incandescence-a phenomenon where an electric current heats a wire filament, causing it to emit photons, the fundamental energy units of light. In the United States, incandescent bulbs began to be phased out in 2007 under the Energy Independence and Security Act. The European Union (EU) implemented a full ban on them starting in 2012, and in 2023, the Biden administration's ban on the manufacturing and sale of incandescent bulbs took effect.
LEDs, by contrast, function through electroluminescence: a process where electronic excitation of a material triggers the emission of photons. The most commonly used material in LEDs is gallium arsenide, though there are numerous variations of this basic compound-such as aluminum gallium arsenide or aluminum gallium indium phosphide. These compounds belong to the "III-V" group of semiconductors, meaning they are formed from elements found in columns III and V of the periodic table. Adjusting the exact composition of the semiconductor can modify the wavelength (and thus the color) of the light it emits. LED emission typically falls within either the visible portion of the light spectrum (i.e., wavelengths ranging from 0.4 to 0.7 micrometers) or the near-infrared range (with wavelengths between 0.78 and 2.5 micrometers). The brightness of the light perceived from an LED depends on two factors: the power the
LED emits and the eye's relative sensitivity to the emitted wavelength. The eye's maximum sensitivity occurs at 0.555 micrometers, a wavelength in the yellow-orange and green region. Most LEDs operate at a relatively low applied voltage, around 2.0 volts, while the current varies based on the application-ranging from a few milliamperes to several hundred milliamperes. The term "diode" refers to the two-terminal structure of this light-emitting device. For example, in a flashlight, a wire filament connects to a battery via two terminals: one (the anode) carrying a negative electric charge and the other (the cathode) carrying a positive charge. In LEDs-much like in other semiconductor devices such as transistors-the "terminals" are actually two semiconductor materials with different compositions and electronic properties, joined together to form a junction. In one material (the negative, or n-type, semiconductor), the charge carriers are electrons; in the other (the positive, or p-type, semiconductor), the charge carriers are "holes"-gaps left by the absence of electrons. When an electric field (provided by a battery, for instance, when the LED is turned on) acts on the junction, current can flow across the p-n junction. This flow creates the electronic excitation that causes the material to emit light.
In a standard LED structure, the transparent epoxy dome plays three key roles: it acts as a structural component to hold the lead frame together, functions as a lens to focus light, and serves as a refractive index matcher to allow more light to escape from the LED chip. The chip-typically measuring 250 × 250 × 250 micrometers-is mounted inside a reflecting cup formed in the lead frame.
Specific material layers determine the LED's emission color: p-n-type GaP:N layers (gallium phosphide with added nitrogen) produce green light; p-n-type GaAsP:N layers (gallium arsenide phosphide with added nitrogen) emit orange and yellow light; and p-type GaP:Zn,O layers (gallium phosphide with added zinc and oxygen) generate red light.
Two major advancements, developed in the 1990s, expanded LED capabilities: LEDs based on aluminum gallium indium phosphide, which emit light efficiently across the green to red-orange spectrum, and blue-emitting LEDs made from silicon carbide or gallium nitride. Blue LEDs can be clustered with other LEDs to create all colors-including white-enabling full-color moving displays.
Any LED can act as a light source for short-range fiber-optic transmission systems, meaning those covering distances of less than 100 meters (330 feet). For long-range fiber optics, however, the light source's emission properties must align with the optical fiber's transmission characteristics-and in this case, infrared LEDs are a better fit than visible-light LEDs. Glass optical fibers experience their lowest transmission losses in the infrared region, specifically at wavelengths of 1.3 and 1.55 micrometers. To match these properties, LEDs are fabricated using gallium indium arsenide phosphide layered on an indium phosphide substrate. The exact composition of this material can be adjusted to ensure the LED emits energy precisely at 1.3 or 1.55 micrometers.
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