Semi-conductor Light Emitting Diodes

The traditional outlets for LEDs have until recent years been in single colour displays, especially alphanumeric ones on instruments, and outdoor billboards and other displays, where they are used in arrays. Another use has been in bright warning lights such as those on the rear of cars and bicycles. However, with the development 

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Al, Ga and In form tetrahedrally coordinated solids with elements of group 15, which are part of the series of III-V semi-conductor (i.e., groups 13-15). The mixed compounds gallium aluminium phosphides Ga r AljP and the arsenide Gaj r AljAs are used for light-emitting diode (LED) displays and semiconductor lasers. 

One active area of research is completely missing. These are the optical and electrical properties, with effects such as the high conductivity of doped conjugated polymers, electro-luminescence in polymeric light emitting diodes, or the ferro- and piezoelectricity of poly(vinylidene fluoride), to cite only a few examples. There is no good reason for this omission, only that I did not want to overload the book with another topic of different character which, besides, mostly employs concepts which are known from the physics of semi-conductors and low molar mass molecules. 

LED s became commercially available in the late 1960 s with the advent of the green-emitting GaP N and the red-emitting GaP Zn,0 diodes. GaP is an indirect band-gap semi-conductor. That is, there is a "trap" that exists near the conduction band that acts as a recombination center for light emission. This mechanism is shown in the following diagram ... 

Although LEDs and laser diodes appear to be similar, in fact- they me made from the same semi-conductor materials, they are very different in that they are designed to behave in much different wa3rs. Tlie light from diode lasers is coherent in contrast to the incoherent light emitted from LEDs. Emission from LEDs result in a 4jr pattern whereas the diode laser emits a very narrow beam. As a result, diode lasers can be used to read the pits on a compact disc or for scanning a barcode in a checkout line, jobs that LEDs cannot perform. 

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