lights on equipment, surgical lights, and medical therapy lights. Continued
enhancements in spectral emission ranges, beam profiles, electrical power con-
sumption, and optical output powers have enabled expansions of LED usage to life
sciences research and biophotonics medical applications in the UV, visible, and
near infrared spectral regions.
4.3.1 LED Operation and Structures
The basic operating principle of semiconductor LEDs is the recombining of
electron-hole pairs, which results in the creation of photons in apnjunction region
[ 11 – 13 ]. A is the interface between n-typepn junctionand p-type semiconductor
materials within a continuous crystal. These materials are created by adding a small
percentage offoreign atoms (calledimpurity atoms) into the regular lattice structure of
pure materials such as silicon. This process is calleddoping. Doping with impurity
atoms that havefive valence electrons produces ann-doped semiconductorby con-
tributing extra electrons. The addition of impurity atoms with three valence electrons
produces ap-doped semiconductorby creating an electron deficiency or ahole. Thus,
in a p-doped region charge transport only occurs in the form of hole conduction,
whereas electron conduction is the charge transport mechanism in an n-doped region.
To achieve a high radiance and a high efficiency, the LED structure must provide
a means of confining the charge carriers and the stimulated optical emission to the
active region of the pn junction where radiative electron-hole recombination takes
place.Carrier confinementis used to achieve a high level of radiative recombi-
nation in the active region of the device, which yields a high efficiency.Optical
confinementis of importance for preventing absorption of the emitted radiation by
the material surrounding the pn junction and for guiding the emitted light out of the
device, for example, into an opticalfiber.
Normally a sandwich layering of slightly different semiconductor alloys is used
to achieve these confinement objectives. This layered structure is referred to as a
double-heterostructure. The two basic LED configurations are the surface emitter
and the edge-emitter. In thesurface emittershown in Fig.4.6, the plane of the
active light-emitting region is oriented perpendicularly to the axis of the optical
fiber into which the light is coupled. In this configuration, a well is etched through
the substrate of the device, into which afiber is then cemented in order to accept the
emitted light. Typically the active region is limited to a circular section having an
area compatible with thefiber-core end face.
For a surface emitter the beam pattern islambertian, as Fig.4.3illustrates, which
means that LEDs emit light into a hemisphere. Consequently, in setups that do not
use an opticalfiber for light delivery, some type of lens is needed for collection and
distribution of the emitted light into a specific pattern for various applications.
A frequently used LED lens design is the total internal reflection (TIR) lens. These
lenses can produce output beams that are collimated or that give a uniform illumi-
nation, for example, of a circular, ring, square, or rectangular shape [ 14 , 15 ].
4.3 Light-Emitting Diodes 99