Session 8 The pn Junction（Ⅱ）

8.1 Breakdown in pn Junction

1. Breakdown

Breakdown in pn junction is the rapid increase of current as the reverse bias voltage is increased beyond some safe limit. Breakdown may result in the destruction of a device but this is not inevitable. Damage is caused by local melting，and if the current is held to a small value by an externalresistance，or is spread over a large area by careful design and manufacture，then the temperature reached are not high enough for Silicon or SiO2 to melt，nor for impurities to diffuse out of their intended positions.

There are two distinct breakdown mechanisms-avalanche breakdownand Zener or tunnel breakdown .

2. Avalanche breakdown

The avalanche breakdown process occurs when electrons and/or holes，moving across the space charge region，acquire sufficient energy from the electric field to create electron-hole pairs by collidingwith atomic electrons within the depletion region. The newly created electrons and holes move in opposite directions due to the electric field and thereby add to the existing reverse-bias current. In addition，the newly generated electrons and/or holes may acquire sufficient energy to ionizeother atoms，leading to the avalanche process. This avalanche process is schematically shown in Fig.8.1（a）. For most pn junctions，the predominant breakdown mechanism will be the avalanche effect.

Fig.8.1（a）avalanche breakdown and（b）Zener breakdown mechanism.

3. Zener breakdown

Zener breakdownoccurs in highly doped pn junctions through a tunneling mechanism . In a highly doped junction，the conduction and valence bands on opposite sides of the junction are sufficiently close during reverse bias that electrons may tunnel directly from the valence band on the p side into the conduction band on the n side，as shown in Fig.8.1（b）.