Power BJT - Reasons of second breakdown

Q.  Which among the below mentioned reasons is/are responsible for the occurrence of second breakdown phenomenon in power BJT?
- Published on 24 Sep 15

a. Large current
b. Distribution of current in a non-uniform manner
c. Excessive power dissipation
d. All of the above

ANSWER: All of the above

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    Discussion

  • Satyawati   -Posted on 05 Oct 15
    As the power transistor exhibits the negative temperature coefficient of resistivity & being the minority carrier devices, they are more prone to the occurrence of thermal runaway. If the current density is non-uniform across the cross-section area and the diameters of current filaments are smaller, then the high value of resistance is offered.

    Hence, the regions associated with these current filaments dissipates maximum power & generates an extensive amount of heat. As BJT is a negative temperature coefficient device, its resistance gets reduced due to the increased level of heating.

    Increasing level of heat ultimately generates the short circuit between collector & emitter by causing a permanent failure of device. Hence, factors such as excessive power dissipation, large current & non-uniform current distribution give rise to the occurrence of second breakdown in power transistors.

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