Sources of Radiation

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Sources of Radiation

• Energy levels
• Ground state
• Maximum energy level
• Photons

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Delta E represents the amount of energy released
that is transferred into a photon
The photon wavelength expression can be
determined if frequency is converted to
wavelength and eV is converted to joules.
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Semiconductor Junction

• Semiconductor atoms organized into shells
  K,L,M, and N.
• Pauli exclusion principle calculates the
  maximum number of electrons that can occupy an
  energy level.
• N1 level - up to 2 electrons
• N2 level up to 8 electrons
• N3 level up to 18 electrons
• Each level can take electrons.

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Semiconductors
Conduction Band
Ec
Ev
Valence Band
Highest energy level at 0K.
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Semiconductors

• 2 types of materials 1. Intrinsic 2. Extrinsic
• Valence band
• Forbidden energy gap
• Conduction band
• Common substrates silicon, germanium
• How is conductivity increased in a semiconductor?
• 1. Apply thermal excitation.
• 2. Increase free electrons by doping method.

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Semiconductors

• Method 2 introduces materials that add
  carriers, either negative ions (excess electrons)
  or positive ions(shortage of electrons).
• Result either n-type or p-type semiconductor.
• Minority and majority carriers p-type holes are
  majority carriers, n-type electrons are majority
  carriers.
• What is covalent bonding?
• Doping materials Arsenic, gallium, boron.

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Semiconductors and Optoelectronics

• Radiation source is a p-n junction
• At p-n junction electrons and holes diffuse
  toward each other and combine.
• This develops a Depletion region.(non
  conducting).
• On the n-type side there is a group of positive
  ions due to electrons diffusing to the p
  side.This creates a Barrier potential preventing
  further diffusion.
• On the p-type side the barrier is negative.The
  two conditions develop a Barrier Voltage.

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Semiconductors and Optoelectronics

• A reverse bias increase barrier voltage. This
  increases the depletion region and current does
  not pass.
• A forward bias decreases the depletion region.
• When forward bias exceeds barrier potential
  current will flow due to electrons and holes
  combining.(recombination)
• Electrons are in the conduction band and at a
  higher level than the holes in the valence band.
  Therefore energy in the form of radiation is
  released.
• When the wavelength is in the visible range it is
  called Electroluminescence.

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Semiconductors and Optoelectronics

• Wavelength of emitted radiation depends
  upon
• Energy gap or barrier voltage between p and n
  determined by semiconductor material.
• Mode of Recombination a.) direct from conduction
  to valence band or b.) trapped in iso-electronic
  centre.
• In both cases the wavelength depends upon the
  energy gap.

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Semiconductors and Optoelectronics

• Energy Gaps
• Silicon 1.09 eV (1140 nm)
• Germanium 0.66 eV (1880 nm)
• Gallium arsenic
• Phosphorus
• Indium
• Antimony

Infrared
Higher energy gaps
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Classification of Radiation Sources by their
Primary Characteristics

• Flux Output
• Wavelength and Colour
• Source Excitation

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Classification of Radiation Sources by Primary
Characteristics

• Flux Output
• Point Source small (narrow) emission
  area.Incidence and illuminance are subject to
  square law effect. Ex. LED, small filament bulb.
• Area Source Large emission area. Ex.light bulb.
  Electroluminescence display.
• Collimated source flux lines are parallel.
  Square law relationship does not apply. Ex.
  Searchlight.
• Coherent source point or collimated source.
  Waves are in phase. Ex. Laser.

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Classification of Radiation Sources by their
Primary Characteristics

• Wavelength and Colour
• Hue blue, red etc. Wavelengthof radiation is
  one of the determining factors.
• Saturation describes the fact that colours are
  not pure single wavelengths. They contain some
  amounts of white light decreasing saturation of
  colour. Ex. Pink is mixture of red and white.
• Intensity flux density of a radiating source or
  the luminance of a reflecting surface.

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Radiation Spectras of Sources

• Continuous Spectrum
• Line Spectrum
• Single wavelength
• Monochromatic

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Classification of Radiating Sources by their
Primary Characteristics

• Source Excitation
• Thermal
• Electroluminescence
• Vacuum fluorescence
• Chato-luminescence
• Lasing

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Radiation Sources

• Incandescent
• Light Emitting Diode
• Electroluminescent
• Vacuum Fluorescent
• Gas Discharge Lamps