Basics of Diodes (Structure & Operation)

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PN Junction Diodes 


  1.    What are they?

For starters, all you need to know for getting started are:
  • Semiconductors like Silicon & Germanium have 4 valence electrons in their normal state.
  • When either a relatively electron rich or electron deficient elements like Boron(3 valence electrons) or Phosphorus(5 valence electrons) are mixed with pure semiconductors (this mixing is formally called Doping), we obtain n-type or p-type semiconductor.
  • n-type being the one with higher electron density due to presence of electron rich element. Similarly p-type being the one with electron deficiency. The electron deficient spots are considered as a positive charge carrier("assumed") and are called holes.
  • Now when a layer(or wafer) of p-type & n-type semiconductors each are joined together, a strange rearrangement of charges takes place along with development of electric field, this bilayered structure is called a PN Junction Diode (a PN junction with two terminals).
A Diode with its symbolic representation.

  2.    How do they operate?

  • As  a PN Junction is formed, its natural to predict positive holes of p side must be attracted to negative electrons of n side. And that's what actually happens, a diffusion current starts flowing from p side to n side (assuming current as the flow of positive charges).
  • But it must be noticed that every electron that leaves the n side leaves behind a hole at its place and vice versa. This means the diffusion current is creating yet another charge separation at the junction of two materials.
  • Also, these newly developed charges are generating an electric field, which is directed from n side to p side. Thus, this electric field is opposing in nature to the drift current which actually created it.





Redistribution of charges with time, upto equilibrium.




  • After a certain amount of time, this field is strong enough that it completely counter acts the initial electric field which caused diffusion current, thereby creating a state of equilibrium at the junction.
  • The newly generated region of separated charges is called Depletion Layer, and it has a potential difference associated with it called Built in potential.
  • Built in potential of a typical Is diode is around 700mV.

Next post will be on Biasing of Diodes!


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