Biasing of PN Junction Diodes

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Biasing of Diodes


  1.    What is biasing? 

Having Understood the structure and equilibrium of a PN Junction diode without any external potential difference applied, its time now to see what happens one the application of an external potential difference:
  • We know diodes are Directional in nature, i.e their polarity matters due to the simple fact that they are not symmetric due to opposite charges on either side.
  • So right off the bat we can predict application of potential can be done in two ways-   1. From n side to p side, 2. From p side to n side.
  • Now, when we apply positive potential to P-side of diode, and of course ground / lower voltage to N-side the resulting configuration is called Forward Bias.
  • Similarly when the opposite is done, resulting configuration is called Reverse Bias.

 

  2.    Forward bias explained

  •  As shown, the external electric field due to the battery points from p side to n side.
  • This field opposes the direction of field in the depletion layer. What this means is that the condition of equilibrium which was created at the junction has yet again been disturbed, which consequently will lead to another current flow.
  • In short, the width of depletion layer decreases, and if the external potential exceeds the built in potential of the junction, the junction will no longer prohibit the flow of charges.
  • The overall behaviour of the diode would be as if its a short circuit! This is forward bias in effect!!
A PN Junction under Forward Bias


   3.    Reverse bias explained


  • For reverse bias, the external cal electric field is coinciding in direction with the depletion layer's electric field.
  • This means that the field which opposes the flow of current through the junction is being further enhanced by our external field.
  • Thus, no current can flow through the junction. And the overall behaviour of diode can be summarised as that of an open circuit.
I-V characteristic of a PN Junction.(Note: current grows exponentially in
forward bias, and is almost negligible in reverse bias)

  

  4.    Zener breakdown

  • Although the diode acts as an open circuit(almost perfectly) for reverse bias, that isn't true for all reverse biasing voltage.
  • If we keep increasing the reverse bias volt, after a certain voltage, what happens is that the electric field established is so large that valence electron (which were not meant for conduction) are pulled apart from their respective atoms and contribute to current flow.
  • This "relatively" high reverse voltage is called breakdown voltage, and this state of the diode is called breakdown region.
  • Now what's special here is that the current established is so large that changes in current(reasonable) don't even affect the potential difference across it, thereby creating the possibility of a voltage regulator.
Notice the sudden growth in current beyond Vbd( breakdown volt)

Next post will be on Models of Diode!



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