IGBT Modules - Technologies, Driver and Application (Second Edition) - page 53

especially at higher currents, as the residual voltageU
in the IGBT itself is less than
in theMOSFET, due to the diffusion voltage created by the pn-junction
). This
behaviour also leads to a simplified equivalent circuit diagram of the IGBT, which
comprises aMOSFETwithPIN diodes in series.
If there is insufficient voltage at the gate, and the inversion layer formed is weak, the
number of electrons that flow into the drift zone is correspondingly smaller. The voltage
drop via the IGBT increases and the IGBT reaches the active area of its characteristic
curve. The significant losses incurred by operating the active area of the IGBT can
destroy the IGBT. Therefore, operation in the active area should be avoided, except
during the switching transients.
Fig. 1.30
IGBToutput characteristics
While the IGBT is turned on, the pn-junction J
remains blocked, because of negative
voltage applied to it. The space-charge region formed between two adjacent cells in this
way pinches off the area between the p-regions at thewidth d
as shown i
This reduces the conductivity of the area which, to a certain extent, determines the
conduction loss of an IGBT. The pinch-off principle is like that for a JFET so the internal
resistance caused by this effect will be referred to in the following as R
. It is present
in all IGBTswith a planar gate structure.
Fig. 1.31
JFET effect between two cells of an IGBT with a planar gate structure (not to
If the gate-emitter voltage is turned off (0V) or inverted, the gate channel re-forms,
preventing an influx of electrons into the drift zone. Because of the high concentration of
charge carriers within the drift zone at this point in time, the electrons are flowing into
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