Difference between a normal PN diode and an LED

Discussion in 'General Electronics Chat' started by Abhinavrajan, Sep 11, 2016.

  1. Abhinavrajan

    Thread Starter Member

    Aug 7, 2016
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    What's the difference between a PN junction diode and a normal LED?

    How does 0.7V or greater voltage given to an LED makes light emit at the junction but PN junction doesnt emit any light when
    Voltage of 0.7V or greater is given?

    Please explain clearly.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    It is all in the materials used to "dope" the silicon. In this context, the verb "to dope" means to introduce impurities into the pure silicon to create an excess or a deficit of valance electrons. These materials are usually from group III or group V of the periodic table. The emission of visible light, or infrared or ultra-violet radiation happens when an excited electron retunrs to a lower energy quantum state. When it does this, it gives up a quantum of energy in the form of a photon. This photon will have a precise and "quantized" energy level. This implies that it will have a precisely define frequency and wavlength. Planck's constant is the constant of proportionality that relates frequency and energy.
     
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  3. Abhinavrajan

    Thread Starter Member

    Aug 7, 2016
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    So, you are telling that materials used in PN junction diode are different than those used in LED?
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    Yes, I am. They both use silicon, what is different is the materials used to dope the silicon. See the following article for a list of the different materials.

    https://en.wikipedia.org/wiki/Light-emitting_diode
     
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  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
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  6. Papabravo

    Expert

    Feb 24, 2006
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    My mistake -- I thought the GaP or GaAs were the dopants, not the semiconductor itself.
     
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  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    UV ("white" LEDs when used with a phosphor, some single color with a 3.3V drop are also likely GaN) are now based on a common material, Gallium Nitride. The GaN is grown by vapor deposition methods on sapphire (most common), silicon (cheap but difficult and thermal issues) or GaN (expensive but very high power density is possible because there is no thermal expansion mismatch - only one company is making these that I know of).
     
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  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    The primary difference in the diodes (and the requirement for different substrates and dopants) is the need for a direct band gap semiconductor for efficient photon generation.
    http://www.doitpoms.ac.uk/tlplib/semiconductors/compound.php

    You can improve the optical characteristics of silicon (like in solar cells or Thermal imaging) with indium doping.
     
    Last edited: Sep 11, 2016
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  9. crutschow

    Expert

    Mar 14, 2008
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    But a silicon junction can emit light, it's just invisible infrared, such as described here.

    The frequency (and thus color) of the emitted light is proportional to the bandgap and forward voltage of the diode, since higher light frequencies have higher energy.
    Thus the 0.7V forward drop of a silicon diodes produces lower frequency infrared light, and the higher bandgap, and forward voltage drop of GaAs LED diodes produces higher frequency visible light.

    You will notice that LEDs that produce the higher frequency colors towards the blue end of the spectrum typically have higher forward drops.
     
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  10. dl324

    Distinguished Member

    Mar 30, 2015
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    Conversely, a silicon junction also exhibits a photovoltaic effect.
     
  11. AlbertHall

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    Jun 4, 2014
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    Ah, yes, I remember the days (1960's) of the poor man's phototransistor. Scrape the paint off an OC71 (germanium PNP, all glass case) and use plasticene to attach it to the inside of a spin dryer (poor man's centrifuge) to move the 'jelly' away from the actual transistor inside.
     
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