IGBT Modules - Technologies, Driver and Application (Second Edition) - page 44

biased in reverse direction between the collector and the base. The weak n
region, also known as the drift zone, now has the task of taking the applied voltage,
together with the p-doped base. Accordingly, the drift zone must be designed with a
sufficient gauge, depending on the voltage range of the transistor. If the maximum
blocking voltage U
(called the breakdown voltage) of the transistor is exceeded,
breakdown occurs and this eventually destroys the component unless countermeasures
are taken from an application viewpoint. The task of the n
-layer is to provide a low-
impedance contact to the collector terminal.
Fig. 1.22
npn-transistor ina common emitter circuit
If a positive voltage greater than the diffusion voltage is connected between the base
and the emitter (U
0.7V for silicon npn-BJTs), the transistor goes into forward
operation. The reason for this is that the electrons sent out by the highly-doped emitter
recombine only partly with holes in the comparatively thin base layer or are diverted to
the base terminal only to a minimal extent. The majority of the electrons can cross
through the base and thus reach the collector. Since the base is p-doped, the holes that
predominate in it are described as majority carriers and the electrons as minority
carriers. Because the collector has greater potential than the base (U
< 0V), the pn-
junction is blocked here for the majority carriers. The collector current is therefore
determined by the minority carriers, while the emitter current is determined by both
types of charge carriers. As long as the voltage between the base and the collector is
negative (i.e. the pn-junction base-collector is blocked for the majority carriers) the
transistor is in theactive areaof theoutput characteristic.
If the transistor is operated with positive base-emitter voltage U
and U
0.7V and
> 0V, both pn-junctionswill be in a conducting state and can inject electrons into the
base. A strong base current I
will flow. In this case, the collector current I
rises sharply
if the collector-emitter voltage U
is increased. The transistor is now in a state of
saturation and the saturation voltageU
is established between the collector and the
emitter. If the transistor is saturated, it requires a certain amount of time to remove the
high concentration of charge carriers from the base, before it can return to the active
state. Because of this delay, the maximum switching speed of a bipolar transistor is
If U
< 0.7V and U
< 0V, both pn-junctions are blocked and there is only a very low
reverse current.
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