Audio Engineering

Audio Amplifi ers 295

in a BJT is the phenomenon known as thermal runaway. This can happen because the
potential barrier of a P-N junction (that voltage that must be exceeded before current will
fl ow in the forward direction) is temperature dependent and decreases with temperature.
Because there will be unavoidable nonuniformities in the doping levels across the
junction, this will lead to nonuniform current fl ow through the junction sandwich, with
the greatest fl ow taking place through the hottest region. If the ability of the device to
conduct heat away from the junction is inadequate to prevent the junction temperature
rising above permissible levels, this process can become cumulative. This will result in
the total current fl ow through the device being funneled through some very small area of
the junction, which may permanently damage the transistor. This malfunction is termed
secondary breakdown, and the operating limits imposed by the need to avoid this failure
mechanism are shown in Figure 9.12. Field effect devices do not suffer from this type of
failure.

9.7 Junction Field Effect Transistors ( JFETs) ............................................................

JFETs are, almost invariably, depletion mode devices, which means that there will be
some drain current at a zero-applied gate-source potential. This current will decrease in a
fairly linear manner as the reverse gate-source potential is increased, giving an operating

Thermal dissipation limit

Secondary breakdown limit

10

1

0.1

0.01

1.0 10

Collector voltage

100

Device voltage limit

(V)

l amperesc

Current limit

Figure 9.12 : Bipolar breakdown limits.