PT IGBTs in parallel. If these IGBTs are not verywell coupled
theremay be significant
differences in current sharing between the individual IGBTs. In extreme cases, this can
cause the entire system to fail, if a single IGBT is overloaded and the process transfers
on to the other IGBTs. In applications inwhich it is intended to usePT IGBTs connected
in parallel, the IGBT is usually classified or preselected according to its saturation
. Manufacturers of PT IGBTs support this by
ranges. Accordingly, only PT IGBTs from the sameU
range should be connected in
parallel, andonly if there is good thermal coupling.
Sample behaviour at different temperature levels of the forward voltageU
a function of the collector current I
1.5.2 Non-punch through (NPT) IGBTs
The next stage in the development of IGBTs was the introduction of the NPT IGBT in
the early 1990s by Siemens (whose former semiconductor department now operates
independently under thename InfineonTechnologies).
Unlike PT IGBTs, here the weak n
-doped base serves as a starting point for the
manufacturing process. This allows the p-doped layer to be made significantly thinner.
Chip gauges between 120 and 200µm are now usual for 1.2kV IGBTs. Furthermore, the
buffer layer commonly required in a PT IGBT becomes superfluous, so that the
electrical field in the blocking state has to be removed within the base region. The path
of the electrical fieldE, which describes a linear decrease toward the collector, is shown
along the base length in
As the field no longer "punches through" the base,
this type of IGBT has become known as "non-punch through". However, due to the
principle of the NPT, the weakly doped n
-basemust bemade relatively thick, as it now
has to take up all the blocking voltage. It therefore represents a significant proportion of
the total losses of the IGBT.
Because the back emitter (p-doped layer) is not as thick, the charge carrier
concentration in this region is not as high as in aPT IGBT. Accordingly, measures to set
the carrier lifetime can be more moderate or become completely unnecessary. The tail
current upon turning off is not as highbut lastsmuch longer, than in aPT IGBT.
A temperature difference of less than 15K prevailing between the baseplate temperatures (in modules) and
package temperatures (in discrete IGBTs) of the IGBTs connected in parallel can be assumed as a reference
value for sufficient thermal coupling.