4.3.2 LED Wavelengths and Device Uses
The operation of semiconductor devices such as LEDs, laser diodes, and photodi-
odes can be understood by considering theenergy-band conceptshown in Fig.4.8.
In a semiconductor the valence electrons of the constituent material atoms occupy a
band of energy levels called thevalence band. This is the lowest band of allowed
energy states. The next higher band of allowed energy levels for the electrons is
called theconduction band. In a pure crystal at low temperatures, the conduction
band is completely empty of electrons and the valence band is completely full. These
two bands are separated by anenergy gap,orbandgap, in which no energy levels
exist. When electrons are energetically excited across the bandgap, a number of
freely moving electrons appear in the conduction band. This process leaves an equal
concentration of vacancies, orholes, in the valence band. Both the free electrons and
the holes are mobile within the material and can contribute to electrical conductivity
when an external electricfield is applied. Light emission takes place when electrons
drop from the conduction band and recombine with holes in the valence band.
The emission wavelengths of LEDs depend on the material composition in the
pn junction because different materials have different bandgap energies, which
determine the wavelength of the emitted light. Common materials include GaN,
GaAs, InP, and various alloys of AlGaAs and of InGaAsP. Whereas the full-width
half-maximum (FWHM) power spectral widths of LEDs in the 800-nm region are
around 35 nm, the widths increase in longer-wavelength materials. For devices
operating in the 1300–1600-nm region, the spectral widths vary from around 70–
180 nm. Figure4.9shows an example for surface and edge-emitting devices with a
peak wavelength at 1546 nm. The output spectral widths of surface-emitting LEDs
tend to be broader than those of edge-emitting LEDs because of different internal
absorption effects of the emitted light in the two device structures.
In biophotonics and medical disciplines, there is a growing use of LEDs in the
UV spectrum ranging from 100 to 400 nm. Care must be exercised when using UV
light because, as noted in Sect.1.3, within the optical radiation spectrum,
Fig. 4.8 Energy band
concept for a semiconductor
4.3 Light-Emitting Diodes 101