From Chromic Phenomena: Technological Applications of Colour Chemistry

3.8 Electroluminescence

Electroluminescence (EL) involves the conversion of electrical energy into non-thermal light. The materials used to achieve this conversion can be either inorganic or organic. Low field devices are the conventional light emitting diodes (LEDs), which are constructed from monocrystalline semi-conducting materials, mostly group III V compounds. Inorganic EL displays are generally high field and are made of insulating materials, where the active material or phosphor is a II VI compound. Organic LEDs are made from either thin films of vapour deposited low molecular weight materials or conjugated polymers put down by spin coating or other means.

3.8.1 Semi-conductor Light Emitting Diodes

Conventional semi-conductor LEDs are made by growing a region of n-type semiconductor material, doped with donors, and p-type semi-conductor material, doped with acceptors, grown as a single crystal. This is shown diagrammatically in Figure 3.26.52 When the material is at equilibrium, the charge in the junction region prevents movement of electrons and holes. By applying a forward bias, or positive charge to the p-type side of the junction electrons are injected into the p-type region and holes into the n-type region, they then combine in the junction region with the majority carriers, leading to emission of light with an energy approximately equal to the band gap. (Note: a material is classified as a semi-conductor when the band gap energy lies in the range of 0.1 6 eV.)

Figure 3.26: Schematic for semi-conductor LED.
(Reproduced with permission of John Wiley & Sons.)

In 1961 it was discovered that gallium arsenide (GaAs)...

Copyright The Royal Society of Chemistry 2001 under license agreement with Books24x7

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Diodes are electronic components that conduct electric current in only one direction, functioning as a one-way valve. Diodes are manufactured using semiconductor materials such as silicon, germanium or selenium and are used as voltage regulators, signal rectifiers, oscillators and signal modulators / demodulators.
General Purpose Diodes
General-purpose diodes are electric components that conduct electric current in only one direction, functioning similarly to a one-way valve.
High Voltage Diodes
High voltage diodes are designed for use in high-voltage applications.
Light Emitting Diodes (LED)
Light-emitting diodes (LED) are PN junction devices that give off light radiation when biased in the forward direction. LEDs are used in a wide variety of indication applications.
Schottky Diodes
Schottky diodes in their simplest form consist of a metal layer that contacts a semiconductor element. The metal / semiconductor junctions exhibit rectifying behavior (i.e., the current passes through the structure more readily with one polarity than the other).

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