IGBT Modules - Technologies, Driver and Application (Second Edition) - page 61

In contrast to aPT IGBTwith its negative temperature coefficient, NPT IGBTs behave in
quite the opposite way. Due to the long carrier lifetime
even at room temperature, an
increase in temperature has very little effect on the increase in charge carriers. In this
case, the reduction in mobility µ of the charge carriers and the increase in contact
resistances of the collector and emitter terminals are more dominant. Likewise, at very
low forward voltage or current, NPT IGBTs also have a negative temperature coefficient.
However, the change from a negative to a positive temperature coefficient happens
when the current is comparable low. The NPT IGBT therefore has a positive
temperature coefficient for the relevant situations under consideration in the application.
Although this causes increased conduction losses as the temperature rises, it also
makes simple parallel connection possible. If anNPT IGBT bears a higher static current
proportion within the parallel connection because of variances, it will heat up
accordingly. This increases its internal voltage drop, which in turn reduces the current.
Consequently, self-regulation is effected by this negative feedback and makes it
possible to carry out connections in parallel at both chip and component level without
having to select IGBTs especially.
Fig. 1.40
Layer sequenceandelectrical field distribution of anNPT IGBT (not to scale)
1.5.3 Fieldstop (FS) IGBTs
The FS
IGBT was developed based on the NPT IGBT concept. Its principal structure
and the path of the electrical field E in a blocking state are shown in
The goal
in development was to furtherminimise the total losses of the IGBT.While then
-base in
NPT IGBTs is still quite thick, the additional fieldstop layer in an FS IGBT means that
the IGBT can be thinner. The role of the FS layer is to remove the electrical field in the
blocking state, in the event that it penetrates the n
-base linearly decreasing if the
voltage gets high enough. In this case, a tapered field distribution similar to that for aPT
IGBT is formed.
As the backside p-emitter in an FS IGBT is not particularly efficient, a relatively weakly
doped n-region can provide the FS layer, so that the efficiency of the backside emitter is
influenced as little as possible. However, the FS layer must be doped sufficiently, in
Besides the name "Fieldstop" (FS) also the following names are used depending on the manufacture: "Soft
Punch Through" (SPT) and "Light Punch Through" (LPT). In the LPT variant, the FS layer is very weak. Its
function therefore comes to the foremore in static blocking than in dynamic blockingof the voltage. The nameFS
is used for Infineon Technologies' IGBTs, while SPT is the usual name for IGBTs from ABB, and Mitsubishi
Electric uses the name LPT.
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