Light emitting diodes (LEDs) have gained broad recognition as the ubiquitous little lights that tell us that our monitors are on, the phone is off the hook or the oven is hot semiconductor. The basic principle behind the emission of light is that: When charge carrier pairs recombine in a semiconductor with an appropriate energy band-gap generates light. In a forward biased diode, little recombination occurs in the depletion layer. Most occurs in a few microns of either P- region or N -region, depending on which one is lightly doped. LEDs produce narrow band radiations, with wave length determined by energy band of the semiconductor.
Solid state electronics have replaced their vacuum tube predecessors for almost five decades. However in the next decade they will be brighter, more efficient and inexpensive enough to replace conventional lighting sources (i.e. incandescent bulbs, fluorescent tubes).
Recent development in AlGaP and AlInGaP blue and green semiconductor growth technology have enabled applications where several single to several millions of these indicator LEDs can be packed together to be used in full color signs, automotive tail lambs, traffic lights etc. still the preponderance of applications require that the viewer has to look directly into the LED. This is not "SOLID STATE LIGHTING"
Artificial lighting sources share three common characteristics:
-They are rarely viewed directly: light from sources are viewed as reflection off the illuminated object.
- The unit of measure is kilo lumen or higher not mille lumen or lumen as it is incase of LEDs
-Lighting sources are pre dominantly white with CIE color coordinates, producing excellent color rendering
Today there is no such commercially using "SOLID STATE LAMP" However high power LED sources are being developed, which will evolve into lighting sources
EVOLUTION OF LEDs
The first practical LED was developed in 1962 and was made of a compound semiconductor alloy, gallium arsenide phosphide, when emitted red light. From 1962, compound semiconductors would provide the foundation for the commercial expansion of LEDs. From 1962 when first LEDs were introduced at 0.001 lm/LED using GaAsP until the mid-1990s commercial LEDs were used exclusively as indicators. In terms of number of LEDs sold, indicators and other small signal applications in 2002 still consume the largest volume of LEDs, with annual global consumption exceeding several LEDs per person on the planet.
Analogous to famous Moore's law in silicon which predicts a doubling of number of transistors in a chip every 18-24 months, LED luminous output has been following Haitz's law, doubling every 18-24 months for past 34 years.