- LED Definition: A Light Emitting Diode (LED) is a semiconductor device that emits light when electric current flows through it.
- Working Principle of LED: The working principle of LED involves applying a forward bias current, which causes electrons and holes to recombine at the junction, producing light.
- Color Determination: The color of an LED depends on the material used and its dominant emission wavelength, which varies with changes in temperature and drive current.
- Efficiency and Dimming: LED efficiency is measured by luminous efficacy, and they can be effectively dimmed using pulse width modulation.
- Applications and Advances: From signaling and display to general and road lighting, LEDs are now widely used due to their improved luminous efficacy and versatile applications.
What is a Light Emitting Diode (LED)?
LED material systems include gallium arsenide phosphide (GaAsP), gallium phosphide (GaP), and aluminum gallium arsenide (AlGaAs). Each material system covers a particular range of photon energies and colors.
An LED produces light by electroluminescence. A forward current injects charge carriers into its active region, where electron-hole recombination can release energy as photons.
Many LEDs use compound semiconductors formed from group III and group V elements. Controlled doping creates the p-type and n-type layers. When a forward current (IF) drives electrons and holes into the active region, the emitted photon energy is set mainly by the band gap (Eg), so material composition determines the wavelength.
How Does a Light Emitting Diode (LED) Work?
When the forward current IF flows through the p-n junction of the diode, electrons move from the n-type side and holes move from the p-type side into the active region. When an electron recombines with a hole, the transition can release a photon.
Radiative recombination releases energy as photons. Non-radiative recombination transfers energy to lattice vibrations or defects and ultimately produces heat. Direct-band-gap semiconductors support efficient radiative recombination, while material defects and other loss paths reduce the light output.
Several metrics describe LED efficiency. Internal quantum efficiency measures photon generation inside the active region, while extraction efficiency accounts for light trapped or absorbed before it leaves the device. External quantum efficiency combines those effects. Wall-plug efficiency compares emitted optical power with electrical input, and luminous efficacy also weights the spectrum for human vision.
Henry J. Round reported light emission from a semiconductor in 1907, and Oleg V. Losev studied the effect in the 1920s and 1930s. Practical LEDs followed later advances in semiconductor materials and junction fabrication. LEDs now serve indicators, displays, signs, indoor lighting and road lighting.
Color of an LED
The color of a narrow-band LED can be specified by its dominant wavelength, λd (in nm). AlInGaP devices commonly cover red, orange and amber, while InGaN devices commonly cover green, blue-green and blue. Exact wavelength bins vary by product. The data sheet specifies color and forward voltage at a stated current and junction or ambient temperature.
Junction temperature and drive current can change an LED’s light output, forward voltage, efficiency and dominant wavelength. Forward voltage usually falls as junction temperature rises, but the direction and size of optical changes depend on the material and device design. Use the manufacturer’s characteristic curves and operate within the stated current and temperature limits.
For a basic electronics project, the best Arduino starter kits include colored LEDs for testing simple current-limited circuits.
Dimming
LEDs can be dimmed by reducing a regulated drive current or by using pulse width modulation (PWM). The usable dimming range, color stability and flicker performance depend on the LED and driver. A compatible driver must keep the current within the device ratings throughout the dimming range.
Reliability
The maximum junction temperature (TJMAX) is an absolute rating, not a normal operating target. Higher junction temperature can accelerate lumen depreciation and other failure mechanisms. LED useful life is often stated as lumen maintenance, such as L70, which is the operating time until output is projected to reach 70% of its initial value. Complete product reliability also depends on the driver, optics, seals and thermal design; MTBF is not the time until half of the LEDs fail.
White LEDs
White LED light is commonly produced in two ways. A color-mixed system combines red, green and blue LED output. A phosphor-converted system uses blue or ultraviolet LED light to excite a phosphor, which absorbs part of that light and emits a broader band at longer wavelengths. The converted and unconverted light combine to appear white.
Luminous Efficacy
The luminous efficacy of an LED source is its emitted luminous flux, in lumens (lm), divided by its electrical input power, in watts (W). Quantum efficiency and optical power efficiency measure different quantities. The maximum spectral luminous efficacy of radiation is 683 lm/W near 555 nm, where photopic human vision is most sensitive, but that value does not describe a practical LED’s electrical efficacy. Actual performance depends on the spectrum, drive current, junction temperature, package, optics and driver, so compare products at the same stated test conditions.
