Physics and technology of mid-infrared light emitting diodes

There is increasing interest in mid-infrared (mid-IR) light emitting diodes which operate in the 2-5 mum spectral region. Efficient LEDs operating at the characteristic absorption wavelengths of target gases, such as CH4, CO2 and CO, have great potential for-the next generation of optical gas sensor s. The fundamental difficulties associated with realizing suitable mid-IR L EDs at different wavelengths, with high continuous wave (CW) output power a t room temperature, relate principally to quantum efficiency and optical ex traction of the light. Each of these will be briefly considered. Some of th e different device designs and techniques used for the suppression of non-r adiative Auger recombination and the reduction of Shockley-Read-Hall centre s are discussed. Liquid phase epitaxy (LPE) continues to hold a strong posi tion in mid-IR LED technology and many of the best LEDs currently available have been fabricated using this technique. In this respect, the LPE growth of associated InAs(Sb)-based epitaxial structures and their purification i s briefly reported. An overview of the 'state of the art' is also given wit h respect to the application of mid-IR LEDs in practical gas sensors.