Light Emitting Diode

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Light Emitting Diode

• From our discussion on radiation sources we
  indicated that all semiconductors produce
  radiation when an electron from the conduction
  band recombines with a hole in the valence band.
• Silicon introduces a drawback. Wavelength is long
  compared to geometry of the substrate and much of
  the radiation is absorbed by the semiconductor
  material.
• An LED requires 1. Semiconductor with a higher
  energy gap. 2. Junction geometry is designed to
  contain radiation.

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Light Emitting Diodes

• gt An LED is typically constructed on a GaP or
  GaAsP n-doped substrate.
• A p-doped layer is epitaxialy grown on the
  substrate.
• Recombination and radiation generation occurs
  between these two layers.
• GaP layer is transparent and radiation will pass
  through the top layer.
• GaP efficiency can be improved with a reflective
  coating placed at the bottom electrode.

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Light Emitting Diode

• A fairly narrow cone of flux will escape from the
  semiconductor (approx. 17 degrees) due to the
  air/GaP interface and application of Snells Law.
  ( TIR)
• Remedy a plastic dome can be introduced to
  increase the angle of departure to approximately
  26 degrees.
• The plastic dome is also part of the package that
  amalgamates the components.
• What does the shape of the dome control?
• The profile of the radiation pattern.
• gt T-1 ¾ and T-1 describe the diameter of the LED
  package in 1/8th inch scale.

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Light Emitting Diode

• Electrical characteristics of an LED
• 1. Breaking voltage ( ) 1.2 to 2 volts
  typical
• 2. Reverse breakdown voltage ( ) 5 volts
  typical
• 3. Maximum power dissipation ( )
• 4. Maximum permissible peak current (
  )

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