- LED Definition: An LED, or Light Emitting Diode, is defined as a semiconductor device that emits light when electrically energized through a process called electroluminescence.
- How an LED works: When forward biased, electrons and holes in an LED recombine at the depletion layer, releasing energy as light, illustrating how an LED works.
- Material Importance: LEDs utilize materials with a direct band gap like Gallium Arsenide and Aluminium Gallium Phosphide, which directly influences the color and efficiency of the emitted light.
- Structural Design: The design of an LED ensures that light is emitted efficiently by positioning the electron-hole recombination zone near the surface and encasing it in a transparent material to reduce loss.
- Applications of LEDs: LEDs are versatile in use, found in products ranging from residential lighting to electronic displays and remote controls, highlighting their broad utility.
A light-emitting diode (LED) is a forward-biased p-n junction diode designed so that electron-hole recombination releases useful light. The semiconductor band gap sets the photon-energy range, while the junction structure, package and drive current affect how much of that light leaves the device.
Working of LED
Forward bias lowers the p-n junction barrier and injects electrons and holes into the active region. When a conduction-band electron recombines with a hole, the released energy can form a photon. Radiative recombination is efficient in a direct-band-gap semiconductor because the conduction-band minimum and valence-band maximum occur at the same crystal momentum.
The diagram shows the band-to-band transition. Real LEDs also have non-radiative recombination paths, so defects, temperature and current density can reduce the fraction of input energy emitted as light. The device package then has to extract photons that would otherwise be absorbed or trapped by internal reflection.
For an ideal radiative transition, the photon energy is approximately E = hν and is related to the semiconductor band gap △ in eV. Here h is Planck’s constant and ν is light frequency. The emitted spectrum has a finite width, so the relation describes the central energy rather than one exact wavelength.
Direct Band Gap
In an indirect-band-gap material, recombination must also exchange crystal momentum with a phonon. Radiative emission is therefore much less probable, and more energy becomes heat through non-radiative paths. Silicon and germanium are indirect-band-gap materials.
Indirect Band Gap
Gallium arsenide (GaAs) is a direct-band-gap compound semiconductor used for infrared emission and related optoelectronic devices. Alloy composition, not doping alone, sets the band gap and much of the emitted colour. Current direct-emitting LEDs commonly use InGaN from violet to green and AlGaInP from yellow to red. Materials historically associated with LED wavelengths include:
- Aluminium gallium arsenide (AlGaAs) for infrared and red emission.
- Gallium arsenide phosphide (GaAsP) for red, orange and yellow emission.
- Aluminium gallium phosphide (AlGaP) for visible emission including green in older material systems.
- Indium gallium nitride (InGaN) for blue, blue-green and near-UV emission.
- Zinc selenide (ZnSe), which was studied for blue emitters but is not the usual material in current commercial blue LEDs.
Physical Structure of LED
An LED places a thin active region between semiconductor layers that inject electrons and holes. Its geometry keeps absorbing material away from the main light path and directs photons toward an emitting surface.
The figure shows two p-n junction arrangements. A transparent window, shaped encapsulant or lens helps light escape the high-index semiconductor and controls the beam. Metal contacts provide electrical connection but are positioned to limit optical blockage.
On many new through-hole LEDs, the longer lead is the anode. Leads may have been trimmed, and other packages use different markings, so the package drawing or datasheet is the reliable polarity reference.
Multi-lead packages can contain two or more LED dies for bi-colour or RGB output. Surface-mount LEDs use pads rather than long leads and are assembled directly onto PCBs. Pin count alone does not identify the internal circuit because common-anode, common-cathode and independently connected packages exist.
An LED must be driven within its rated forward current and junction temperature. Indicator LEDs often operate at milliamps, while lighting packages may use much higher current. Forward voltage depends on material, colour, current and temperature. A series resistor can limit current in a simple circuit; regulated lighting products normally use a constant-current driver sized from the device datasheet.
White Light LEDs or White LED Lamps
An individual LED emits over a limited spectral band rather than producing white light directly. White output can come from colour mixing, phosphor conversion or a hybrid of both. The driver, phosphor, optics and thermal design all affect the efficacy and colour of a finished lamp. Two basic methods are:
- Mix red, green and blue LED output in controlled proportions. The system can tune colour, but the result depends on optical mixing and drive control.
- Use a phosphor on or near an LED so that part of the source light is converted to longer wavelengths. A blue InGaN LED with a yellow-emitting phosphor is a common commercial example.
Applications of LEDs
- Electronic displays. OLEDs, micro-LEDs and quantum-dot displays use distinct emitter or conversion structures, so these names are not interchangeable.
- Status and fault indication in electronic equipment.
- Infrared transmitters in remote controls.
- General, architectural, vehicle and street lighting.
- Opto-isolators, where an LED transfers a signal across an electrical isolation barrier.
