PN junctions and diodes

Why are termination structures such as guard rings placed at the edge (periphery) of a high-voltage PN junction?

  1. ATo make the junction emit light
  2. BTo prevent the electric field from concentrating at the junction edge and causing breakdown at a lower voltage than designed
  3. CTo increase the junction capacitance
Show answer and explanation

Answer: B. To prevent the electric field from concentrating at the junction edge and causing breakdown at a lower voltage than designed

Real junctions are finite, and their curved edges concentrate the electric field, so they break down at a lower voltage than the flat part. Termination structures such as guard rings and field plates relax the field at the edge so that the designed breakdown voltage is achieved. This is especially important in power devices.

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