What is Light Emitting Diode?
The light emitting diode is a special type of PN junction diodes. The light emitting diode is specially doped and made of a special type of semiconductor. This diode can emit light when it is forward biased.
Aluminum indium gallium phosphide (AlInGaP) and indium gallium nitride (InGaN) are two of the most commonly used semiconductors for light emitting diode (LED) technologies. They have replaced the older LED technologies which used gallium arsenide phosphide (GaAsP), gallium phosphide (GaP), and aluminum gallium arsenide (AlGaAs). Light-emitting diodes (LEDs) generate visible radiation by electroluminescence phenomenon when low-voltage direct current is applied to a suitably doped crystal containing a p-n junction as shown in Fig. 1. The doping is typically carried out with elements from column III and V of the periodic table. When a forward biased current, IF
, energizes the p-n junction, it emits light at a wavelength defined by the active region energy gap, Eg
When the forward biased current IF
is given to the p-n junction, minority carrier electrons are injected into the p-region and corresponding minority carrier electrons are injected into the n-region. Photon emission occurs due to electron-hole recombination in the p-region. Electron energy transitions across the energy gap, called radiative recombinations, produce photons (i.e., light), while shunt energy transitions, called non-radiative recombinations, produce phonons (i.e., heat). The luminous efficacies of typical AlInGaP LEDs and InGaN LEDs for different peak wavelengths are shown in the table below.
The efficacy depends on the light energy generated at the junction and losses due to re-absorption when light tries to escape through the crystal. The high index of refraction of most semiconductors causes the light to reflect back from the surface into the crystal and highly attenuated before finally exiting. The efficacy expressed in terms of this ultimate measurable visible energy is called the external efficacy.
The phenomenon of electroluminescence was observed in the year 1923 in naturally occurring junctions, but it was impractical at that time due to its low luminous efficacy in converting electric energy to light. But, today efficacy has increased considerably and LEDs are used not only in signals, indicators, signs, and displays but also in indoor lighting applications and road lighting applications.
The color of an LED device is expressed in terms of the dominant wavelength emitted, λd (in nm). AlInGaP LEDs produce the colors red (626 to 630 nm), red-orange (615 to 621 nm), orange (605 nm), and amber (590 to 592 nm). InGaN LEDs produce the colors green (525 nm), blue green (498 to 505 nm), and blue (470 nm). The color and forward voltage of AlInGaP LEDs depend on the temperature of the LED p-n junction. As the temperature of the LED p-n junction increases, the luminous intensity decreases, the dominant wavelength shifts towards longer wavelengths, and the forward voltage drops. The variation in luminous intensity of InGaN LEDs with operating ambient temperature is small (about 10%) from − 20°C to 80°C. However, the dominant wavelength of InGaN LEDs does vary with LED drive current; as the LED drive current increases, dominant wavelength moves toward shorter wavelengths.
LEDs may be dimmed to give 10% of their rated light output by reducing the drive current. LEDs are generally dimmed using Pulse Width Modulation techniques.
The rated maximum junction temperature (TJMAX) is the most critical parameter for an LED. Temperatures exceeding this value usually result in damage of the plastic encapsulated LED device. Mean Time Between Failures (MTBF) is used to find out the average life for LED. MTBF is determined by operating a quantity of LED devices at rated current in an ambient temperature of 55°C and recording when half the devices fail.
White LEDs are being manufactured now using two methods: In the first method red, green, and blue LED chips are combined in the same package to produce white light; In the second method phosphorescence is used. Fluorescence in the phosphor that is encapsulated in the epoxy surrounding the LED chip is activated by the short-wavelength energy from the InGaN LED device.
Luminous efficacy of LED is defined as the emitted luminous flux (in lm) per unit electrical power consumed (in W). Blue LEDs have a rated internal efficacy in the order of 75 lm/W; red LEDs, approximately 155 lm/W; and amber LEDs, 500 lm/W. Taking into consideration losses due to internal re-absorption, the luminous efficacy is on the order of 20 to 25 lm/W for amber and green LEDs. This definition of efficacy is called external efficacy and is analogous to the definition of efficacy typically used for other light source types.