IV.23 - Non-silicon LEDs?

This section was contributed by Kathy Meehan, meehan@srvr.third-wave.com.

Light Emitting Diodes (LEDs) are very similar to semiconductor lasers. They just never reach the population inversion necessary for lasing. The reasons for this vary. The device might have an absorbing region along the light path or the Q of the cavity may have been destroyed by coating the mirrors with antireflective coatings in the case of edge emitting diodes (ELEDs) or superlumineant diodes (SLEDs). Or, there may not be any optical cavity created as in the case of a surface emitting diode (which some people also call SLEDs - watch out for these acronyms, they don't always refer to what you think). The short wavelength LEDs may emit light via a recombination from a trap or dopant level to valence band. This mechanism is what enables GaP and SiC to emit light. Other unusual recombination mechanisms are involved with polymer LEDs.

LEDs have a wider spectral output compared to semiconductor lasers. The spectral width of an SLED range from 25-40nm, of an ELED from 30nm-80nm and of a surface emitter from 50nm-150nm, depending on emission wavelength, current density (amount of current is flowing per unit area) and device structure. LEDs are slower than lasers as the rise and fall times are dependent on a combination of the spontaneous and nonradiative carrier lifetimes, not the stimulated carrier lifetime. The output power of an LED is determined by a number of parameters which include the active area volume and doping level, the injected current, heat sinking (packaging) and operating temperature. The advantages of using an LED over a semiconductor laser are include lower optical noise, greater reliability, less sensitivity to optical feedback, lower cost and no eye safety regulations (thus far). Uses include automotive taillights, billboards, printers and sources for LANs.

References:

• Light Emitting Diodes by P. Dean and ???
• Handbook on LEDs, Hewlett-Packard Publication

Companies: See also The Laser-Focus Buyers Guide or The Photonics Directory, Laurin Publication

Display LEDs (visible)

• Dialight, 1913 Atlantic Avenue, Manasquan, NJ 08736 (908) 223-9400
• Nichia, Japan - AlInGaN LEDs
• Cree Research, N.C. SiC and GaInN LEDs

Telecommunication LEDs (1.3um and 1.5um):

• AMP-Lytel, 61 Chubb Way, Somerville, NJ (908) 685-2000
• Lasertron - no longer in business
• Laser Diode, 4 Olsen Avenue, Edison, NJ 08820 (908) 549-9001

LED Arrays

• Rohm, 3034 Owen Drive, Antoch, TN 37013 (615) 641-2020
• NIR LEDs (1.65 - 3um)
• Telcom Devices, 829 Flynn Drive, Camarillo, CA 93012-8702 (805) 445-4500

Associated Technologies

• Micro-Optics, Nippon Sheet Glass, Big Sky, Corning