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L.E.D. Labs
Background
• Light is transmitted as electromagnetic
waves in tiny packets called photons.
• The energy associated with each photon is
inversely proportional to its wavelength- or
directly proportional to its frequency.
• Therefore, the higher the frequency
producing the light, the more the energy of
the wave.
Incandescent vs. LED
• Incandescent bulbs produce light based on
a filament reaching a temperature above
500 ºC.
– If the filament is hot enough, it will produce a
range of frequencies, thus they can emit white
light.
• LED produce light as electrons change
energy levels.
– The color of light emitted from an LED
depends on the atoms used to create the LED.
Electron Movement in Solids
Electrons move toward the positive electrode in gaps while the
electron hole moves toward the negative electrode.
Isovalent
• Solids with the same average number of
valence electrons per atom
• Gallium arsenide (GaAs) is isovalent with
germanium.
Band Gap Energy
• The amount of energy to free (mobile) an
electron in a solid semiconductor.
• The band gap energy corresponds to the
difference between the ground state (two
electron bond) and the excited free
electron.
Light Production
• Recall, light is produced as an electron
returns to its ground state.
• The frequency of light produced varies
with the change in energy as the electron
drops back to its stable energy level.
Composition & Light Frequency
• Because the band gap energy varies with
the semiconductor composition, the
frequency of light emitted changes with the
composition.
Bond Length & Energy
• Longer bonds are weaker bonds which
require less energy to ionize the electron.
• Therefore, larger atoms having longer
bonds will emit a lower frequency light.
Dopants
• Impurities added to semiconductors as a
method of controlling the number of mobile
electrons.
• Make up only 1: 1,000,000 atoms.
Semiconductor Types
• n-Type: a semiconductor with an excess
amount of electrons because a donor,
dopant with one extra electron, is used.
• p-Type: a semiconductor with too few
electrons because an acceptor, dopant
with one fewer electrons, is used.