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Semiconducting LightEmitting Devices
James A. Johnson
16 December 2006
Semiconducting Materials
Direct Semiconductors
Indirect Semiconductors
Conduction band exists directly
“above” valence band.
Electrons and holes may recombine
without diverting energy to maintaining
conservation of momentum.
Excess energy resulting from
recombination is converted to photons.
Prime example of direct
semiconductor material is Gallium
Arsenide (GaAs)
Conduction band and valence bands are
offset by some vector k.
In order for electrons and holes to
recombine, they must travel across the kspace.
Traveling across k-space requires
conservation of energy and momentum.
Energy that would have become photons in
direct semiconductor is used for travel.
Conservation of momentum is attained by
production of phonons.
Prime example of indirect semiconductor
material is Silicon (Si)
Light-Emitting Device Structure
Four layers in the basic
Light-emitting device
structure: Substrate, ntype material, active
region, p-type material.
Substrate is typically
constructed from n-type
Si or Sapphire.
n-type layer is typically
a GaAs or GaN based
alloy.
Active region is typically
p-type GaAs or GaN.
p-type layer is typically
a p+-type GaAs or GaN
based alloy.
Common gallium based alloys include:
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Indium Gallium Arsenide - InGaAs
Aluminum Gallium Arsenide - AlGaAs
Indium Gallium Nitride - InGaN
Aluminum Gallium Nitride - AlGaN
Basic Light-Emitting Device Types
Edge Light-Emitting
Device
Edge emitting device structure lends
itself to use current fabrication
technologies with little modification.
Double heterojunction structure
provides greatest efficiency by creating
waveguides for new photons.
Laser diodes have highly polished
surfaces for focused light emission and
roughened surfaces to minimize
reflection.
Basic Light-Emitting Device Types
Improving Efficiency in the Edge LightEmitting Device
Homojunction – Low internal reflection.
Photon energy is absorbed and lost in
the surrounding semiconductors.
Single Heterojunction – Improves
efficiency of light emission. Energy is
still lost in the homogenous
semiconductor layer.
Double Heterojunction – Greatest
efficiency. Both semiconductor layers
reflect photons and guide them to the
edge of the structure.
Basic Light-Emitting Device Types
Surface Light-Emitting
Device
Surface emitting device is fabricated using
current technologies, but process is more
complicated as all layers do not have
uniform areas.
Multiple wavelengths may be constructed in
the same device.
Light with longer wavelengths are
constructed deeper in the device.
All electrical contacts are placed on the
same side; rather than “sandwiching” the
device as observed in the edge emitter.
Allows better control of directionality of
emitted light.
Challenges with Light-Emitting
Devices
Edge Emitting
Devices
Surface Emitting
Devices
It is difficult to construct
2-dimensional arrays of
edge-emitting devices.
Packaging must be used
to improve directionality
of light emission.
Fabrication requires
etching of portions of
layers to place electrical
contacts.
Due to placement of the
electrical contacts, it is
difficult to guarantee
uniform current in each
layer of the device.
Light-Emitting Device Applications
and Packaging
A Sample of Applications
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PCB Components
Traffic Lights
Motor Vehicle Lights
Railroad Crossing Bars
Flashlights
Remote Controls
Christmas Decorations
Fiber Optic Transmitters
Laser Pointers
Packaging depends greatly upon
the application of the device.
Traditional through-hole and
SMD components use reflective
cups to deflect emitted light.
Devices are being redesigned to
accommodate more efficient
packaging such as epitaxial liftoff.
Surface emitting devices are
being designed to have lenses
fabricated on the emitter area to
focus or disperse light and to
interface with fiber optic cables.
Emerging Light-Emitting Devices
LED’s using quantum dots.
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This LED combines the edge emitting
structure with the efficiency of surface
emitting devices.
A layer of CdSe/ZnS core-shell
nanocrystal quantum dots (NQD’s) is
placed on top of the light-emitting device
structure.
NQD’s have exceptional luminescent
properties, but are difficult to energize.
Wavelength may be tuned at time of
fabrication based on the size of the
NQD’s.
NQD’s are energized when
recombination energy is transferred to
the quantum dots through a process
called Förster Energy Transfer.
This offers a practical solution to the
problem of general lighting sources
suitable to illuminate objects from large
cities to optical microscopes due to
efficiency, stability, expected longevity,
and potential ease of fabrication.