Class 12 Physics Semiconductor Electronics p-n junction formation

p-n junction formation

  • A p-n junction is the basic building block of many semiconductor devices like diodes, transistor etc.

 

  • Consider a thin p-type silicon semiconductor wafer. Convert a part of the p-type semiconductor into n-type silicon semiconductor by adding a small quantity of pentavalent impurity
  • The holes are the majority carriers in the p-type semiconductor and electrons are the majority carriers in the n-type semiconductor

  • In n-type semiconductor, the concentration of electrons is more compared to the concentration of holes. Similarly, in p-type semiconductor, the concentration of holes is more compared to the concentration of electrons
  • The first process that occurs in the p-n semiconductor is diffusion
  • In the formation of the p-n junction, due to the concentration gradient across the p and the n sides, the electrons diffuse from n region to p region and the holes diffuse from p region to n region

  • Diffusion current –
    • The motion of charge carriers due to the difference in concentration in two regions of the p-n junction, across the junction gives rise to diffusion current
  • As the diffusion continues, it leaves behind a positive charge on the n-side close to the junction. This positive charge, also called as ionised donor is immobile due to bonding with the surrounding atoms
  • Similarly, in the p-region, close to the junction, there is a negative charge or acceptor ions which are immobile

 

  • Depletion region formation –
    • The space charge region on both sides of the p-n junction has immobile ions and is also devoid of any charge carriers
    • This results in the formation of depletion region near the junction
  • Field setup –
    • The depletion region formation results in setting up a field at the junction
    • The field set up along the junction acts like a fictitious battery connected across the junction with positive terminal connected to the n-region

  • Barrier creation –
    • The electric field at the junction sets a barrier which opposes further diffusion of majority charge carriers through the junction
    • Thus, the barrier gets created at the junction prevents further diffusion
    • Width of the barrier – The physical distance from one side of the barrier to the other is called the width of the barrier
    • Height of the barrier –The difference in potential from one side of the barrier to the other side is known as the height of the barrier
  • Drift current
    • Due to the electric field developed at the junction, the electrons from the p-region move to the n-region. Similarly, the hole from the n-region move to the p-region. This results in drift current
    • The motion of charge carriers across the junction due to the electric field is called drift. This results in drift current
  • Drift current Vs Diffusion current
    • The drift current is in a direction opposite to that of the diffusion current
    • At a particular stage, the drift current becomes equal to the diffusion current
    • This stage is set to be equilibrium state when no current flows across the p-n junction
    • Potential barrier becomes maximum and is equal to VB
  • Thus, a p-n junction is formed. Thus, in a p-n junction under equilibrium there is no net current
  • The diagram below shows the p-n junction at equilibrium. . The n-material has lost some electrons and the p-material has acquired the electrons

  • Thus the n material is positive with respect to p material. The potential also prevents the movement of electron from the n-region to the p-region. This potential is called the barrier potential and is indicated as

  • When doping concentration is small, the electrons or holes move a large distance before collision with another electron or hole
  • Hence, the width of the p-n junction becomes large
  • As width of p-n junction increases, the electric field becomes small

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