Feb 16, 2015

PN Junction Diode



If a function is formed between a sample of P-type semiconductor and N type semiconductor, and this device called the PN junction or junction diode possess the properties of a rectifier.  

Open Circuited PN Junction:

The figure below represents two blocks of semiconductor material, one P type and other N type. In the P type material the small circles represent holes, which are the majority charge carries in that type of material.
P type semi conductorN type semi conductor

The dots in the N type material represent the majority charge carrier free electrons within that material. Normally the holes are uniformly distributed throughout the volume of the P type, and the electrons are uniformly distributed in the N type semiconductor.

  • In the above figure P type and N type semiconductor materials are shown side by side, representing a PN junction. Because holes and electrons are close together at the junction, some free electrons from the N side are attached across the junction to fill holes on the P side.
  • They are said to diffuse across junction, i.e. flow from the junction create negative ions on the P side by giving some atoms one more electron than their total number of protons. The electrons also leave positive ions behind them on the N side.  
PN junction diode electron holes crossing
  • When a P type semiconductor is suitably joined to N type semiconductor the contact surface is called PN junction.

Formation of PN Junction

  1. The process involved in the formation of a PN junction are summarized below.
  2. Holes from the P side diffuse into N side where they combine with free electrons.
  3. Free electrons from N side diffuse into the P side where they combine with holes
  4. The diffusion current also known as recombination current delays exponentially both with time and distance from the junction.
  5. Due to the departure of free and mobile carriers from both sides of the junction, a depletion layer is formed. This layer contains only immobile or fixed ions of opposite polarity.
  6. These uncovered by fixed ions setup a potential barrier across the junction.
  7. This potential difference opposes the diffusion of free majority charge carriers from one side of the junction to the other till the process is completely stopped.
  8. The width of the depletion layer depends on the doping level. For heavy doping, the depletion layer is physically thin because a diffusing charge carrier (either from electron or holes) has not to travel far across the junction for recombination.


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