IGBT Modules - Technologies, Driver and Application (Second Edition) - page 59

Fig. 1.38
Layer sequenceandelectrical field distribution of aPT IGBT (not to scale)
With blocking voltage applied, the electrical field widens only in a short area within the
emitter p-area, due to the sharp differential in doping between the p- and n
Instead, it fully penetrates the base and does not decay until the strongly n
buffer layer, hence the name "punch through", as the field punches through the base if
the voltage is high enough. However, the field does not extend as far as the p
so does not represent punch through behaviour in the true sense of the term.
Nevertheless, the name has become established for this type of IGBT. Thanks to the n
buffer layer, the base region can be shortened. The buffer layer also has a second task:
Some of the holes emitted by the p
-layer recombine in the buffer layer. This reduces
the inherent efficiency of the p
-emitter, which counteracts the turning-off behaviour with
a short tail current and a short fall time for the IGBT. The buffer layer influences the
trade-off behaviour of the IGBT by regulating the relationship between the conduction
loss and the switching losses.
In general, the semiconductor substrate has a negative temperature coefficient since,
when the temperature increases, the intrinsic charge carrier concentration n
as shown in
This decreases the resistance of the semiconductor. By carrying
out measures to set the carrier lifetime, often a positive dependence to temperature is
superimposed. Depending on exactly how the doping and lifetime settings have been
adjusted, the IGBT displays either a mainly positive or a mainly negative temperature
coefficient towards the outside. IGBTs usually have a point of alternation at which they
change their behaviour.
PT IGBTs have a low carrier lifetime
at room temperature. This carrier lifetime
increases as the temperature increases, so that more charge carriers are available, i.e.
the resistance of the PT IGBT decreases as the temperature rises. This effect is partly
masked by reducedmobility µ of the charge carriers and a rise in junction resistance at
the collector and emitter terminals. In summary for PT IGBTs the forward voltage U
reduces at the same current, as the temperature increases. This means that they have
a negative temperature coefficient and, in many cases, the change from a negative
temperature coefficient to a positive temperature coefficient does not occur until the
nominal current is exceeded. In practice, this makes it more difficult to connect several
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