Page 143 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

Light-Emitting Diodes 137

are currently being made from both semiconductor p-n diodes and
polymer structures. The light emission principle for LEDs is the re-
combination of excess concentrations of electrons and holes. Semicon-
ductor LEDs and polymer LEDs differ primarily in the physics of how
the excess carrier concentrations are created.
Measurement and device characterization methods of LEDs are
largely independent of the materials used to fabricate the devices.
The principal performance specifications are spectral response, effi-
ciency, and modulation bandwidth. Brightness, which we have not
discussed in this chapter, is a measure of the light intensity gener-
ated per unit area. High brightness is not particularly useful for dis-
play applications. A very high brightness LED may begin to resem-
ble a point source. The light from such an LED could be coupled
more efficiently into an optical fiber with core dimensions compara-
ble to the wavelength of light, such as a single-mode fiber.
The light intensity emitted from an LED is peaked at an energy
that is closely related to the band gap of the material. The spectral
shape of the emission is affected at higher energies through self-
absorption by the emitting material and secondarily through Boltz-
mann statistics that remind us that the number of higher energy elec-
trons and holes that are available to recombine falls off exponentially
with increasing energy.
Fresnel’s law of reflection and refraction at the interface between
two materials—in this case between a semiconductor and air—shows
that only 2% of the total emitted light can escape through one surface
of a diode structure that has smooth faces. However, semiconductor
wafers with smooth faces are also low-emissivity structures. Two
structures were discussed that have been shown to improve the exter-
nal quantum efficiency. One structure is a lens. The other structure
increases the emissivity of the surface.
Modulation bandwidth is determined by limitations of the external
circuit and the internal response of the LED recombination process.
In most cases, the bandwidth of an LED will be determined by ex-
ternal factors, namely the series resistance of the LED and its ca-
pacitance in forward bias. The bandwidth due to the diode materials
properties depends principally on the carrier concentration. There
are two ways to increase this concentration: high injection or high
doping. The easiest way to determine which situation holds is to
measure the rise time or fall time of the LED as a function of the
drive current. If the rise time is independent of the drive current,
then the diode is in the high-doping regime. If the rise time gets
shorter as the drive current increases, then the diode is in the high-
injection regime.

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