Second breakdown in the forward and reverse base current region

At a constant forward or reverse base current, the pulse length of a collector current pulse, which forces a transistor under test into second breakdown, is measured. For all transistors measured, the pulse length
\log \Delta t
is plotted vs. the collector pulse height, with the forward or reverse base current as a parameter. According to our observations, one can distinguish three types of transistors, as far as their secondary breakdown characteristics are concerned. 1) Good epitaxial transistors show an approximately linear dependence of
\log \Delta t
to I
c
with a negative slope. The curves for reverse base current are displaced parallel to the ones for forward base current, the energy of the pulses not being much different in both cases. 2) Bad epitaxial transistors show an S-shaped curve of
\log \Delta t
vs. I
c
for reverse base currents. A deep minimum in second breakdown energy is observed at low collector currents. These transistors will readily fail in practical circuits, where the base current is allowed to reverse. 3) With homogeneous material, the
\log \Delta t
vs. I
c
curves for forward and reverse base currents practically coincide. This method has revealed that a large number of transistor types, including overlay structures, show a decrease of energy handling capability of two to three orders of magnitude at collector currents of about 100 mA when the base current is reversed. It has also proved to be an indispensable tool in the development of high-frequency high power devices, which combine good electrical and second breakdown properties.