Abstract:
A major problem in operating epitaxial bipolar transistors in
reverse biased condition 18 the occurrence of current mode
second breakdown. The base current reversal may be brought about
by the conditions: i) operation above open-base breakdown
voltage BVCEO and ii) during turn-off, where charge is being
extracted from the base-emitter region. The field distribution
within the collector arising from the mobile space charge causes
voltage collapse accompanied by an internal current constriction
and this, in turn, can very rapidly produce damage to the
device. In the case of power transistor switch driven with an
inductive load at the end of the storage time the magnitude of
the collector-enlitter voltage increases rapidly until at some
critical voltage and simultaneously at some critical current
density, avalanche injection occurs at the n~n+ interface. These
two parameters are very important ln determining the
susceptibility of a device to failure by current mode second
breakdown. Therefore, there is a need to know the dependence of
the critical voltage and current density upon the collector
doping density and epitaxial layer thickness in order to
determine the resistance of power transistor to current mode
second breakdown and also, to design protective circuit. The
study permits the evaluation of critical current density and voltage of bipolar power transistor switches with inductive
loads. Results show the dependence of this two important
parameters upon collector doping density and epitaxial layer
thickness.
