Emitting Diodes LED

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  

More recently, the advent of improved LED devices, emitting red, orange, green, yellow and blue emission have become available. The progress made is shown in the following diagram, shown on the next page  

As can be seen, a wide variety of energy gaps exists for these semiconductors. However, it is the electron-mobility which is the deciding factor In the selection of materials for LED s. That and their relationship to the energy required for visible emission. These include ZnSe, AlP, GaP, AlAs, AlSb, CdTe. GaAs, and InP. However, AsAs. GaP and AlSb are indirect band-gap semi-conductors and have not been found useful for high brightness LED s. 

The family of III-V and II-VI semi-conductors has a wide-range of lattice constants and corresponding band-gaps. Those with smaller lattice 

Semi- Conductor Lattice Const A Energy Gap -electron volt Electron Mobility Hole Mobility  

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AQGa As grown on GaAs is used for the preparation of light-emitting diodes (LEDs), injection lasers and... 

A light-emitting diode (LED) is a forward-biasedp—n junction in which the appHed bias enables the recombination of electrons and holes at the junction, resulting in the emission of photons. This type of light emission resulting from the injection of charged carriers is referred to as electroluminescence. A direct band gap semiconductor is optimal for efficient light emission and thus the majority of the compound semiconductors are potential candidates for efficient LEDs. 

Finally, an electric current can produce injection luminescence from the recombination of electrons and holes in the contact 2one between differendy doped semiconductor regions. This is used in light-emitting diodes (LED, usually ted), in electronic displays, and in semiconductor lasers. 

By 1988, a number of devices such as a MOSFET transistor had been developed by the use of poly(acetylene) (Burroughes et al. 1988), but further advances in the following decade led to field-effect transistors and, most notably, to the exploitation of electroluminescence in polymer devices, mentioned in Friend s 1994 survey but much more fully described in a later, particularly clear paper (Friend et al. 1999). The polymeric light-emitting diodes (LEDs) described here consist in essence of a polymer film between two electrodes, one of them transparent, with careful control of the interfaces between polymer and electrodes (which are coated with appropriate films). PPV is the polymer of choice. 

The main use of elemental As is in alloys with Pb and to a lesser extent Cu. Addition of small concentrations of As improves die properties of Pb/Sb for storage batteries (see below ), up to 0.75% improves the hardness and castabilily of type metal, and 0 5-2.0% improves the sphericity of Pb ammunition. Automotive body solder is Pb (92%),, Sb (5 0%), Sn (2.5%) and As (0.5%). Intcrnxitallic compounds with Al, Ga and In give the 111-V semiconductors (p 255) of which GaAs and InAs are of particular value for light-emitting diodes (LEDs), tunnel diodes, infrared emitters, laser windows and Hall-effect devices (p. 258). 

LC state. See Liquid crystalline (LC) state LED devices. See Light-emitting diode (LED) devices... 

Optoelectronic components produced by CVD include semiconductor lasers, light-emitting diodes (LED), photodetectors, photovoltaic cells, imaging tubes, laser diodes, optical waveguides, Impact diodes, Gunn diodes, mixer diodes, varactors, photocathodes, and HEMT (high electron mobility transistor). Major applications are listed in Table 15.1.El... 

Optoelectronic and Ferroelectric Applications 389 3.1 Light Emitting Diodes (LED)... 

Figure 15.2. Schematic of typical light-emitting diode (LED).

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