Reverse breakdown (PN Junction )

  • When a PN junction is reversed biased it allows very small current to flow through it. This current is due to the movement of minority charge carriers and it is almost independent of the voltage applied.
  • If reverse bias is made too high, the current through PN junction increases abruptly and the voltage at which this phenomenon occurs is called breakdown voltage . 
  • At this breakdown voltage , the crystal structure breaks down.
  • This crystal structure returns to the normal state when when excess reverse bias is removed , provided that overheating has not permanently damaged the crystal.
  • There are two processes which causes junction breakdown. One is zener breakdown and another one is avalanche breakdown . 
  • When reverse bias is increased, the electric field at the junction also increases. High electric field causes covalent bonds to break resulting in generation of large number of  charge carriers. This causes a large current to flow. This mechanism of breakdown is called  zener breakdown .
  • In case of avalanche breakdown , the increased electric field causes increase in the velocities of the minority carriers. These high energy carriers break covalent bonds, thereby generating more carriers. Again these generated carriers are accelerated by electricfield. They break more covalent bonds during their travel. A chain is thus established, creating a large number of carriers. This gives rise to a high reverse current. This mechanism of breakdown is called  avalanche breakdown . 

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