IGBT Modules - Technologies, Driver and Application (Second Edition) - page 65

Fig. 1.45
andTrench-FS IGBT compared (not to scale)
In a conventional Trench-FS IGBT, the holes sent from the highly p-doped collector side
to the n
-base reach the emitter directly. In a CSTBT
, a further mechanism is
implemented. The n-dopedCS layer has a slightly higher doping concentration than the
weakly doped n
-base. Accordingly, the potential between theCS layer and the p-region
on the emitter side is greater than the potential in relation to the n
-base if therewere no
CS layer. The higher voltage serves as a barrier for the holes emitted by the p-doped
collector, forming a zone enhanced by holes, which creates a boost to the charge
carriers near the emitter side. In other words, the potential that expands within the CS
layer stores the holes sent out by the back emitter (p-layer) near the emitter at the front
surface, increasing the charge carrier density. This has a positive effect on the
saturation voltageU
1.5.6 IEGTs
Toshiba presented the IEGT (injection enhanced gate transistor) as a substitute for gate
turn-off thyristors (GTOs) in 1993, basing it on high-voltage IGBTs. The principle of the
IEGT is adapting the geometry of the trench gate structure, which is achieved by an
accumulation layer of holes below the p-region of the emitter side (crosshatched area in
. This is achieved by having the gates pinch the area of the n
-base. The holes
present there, coming from the collector side, move within this region only by diffusion
and the entire hole current is limited. The hole current is a function of the distance w
between the gates and the height d of the area of the n
-base enclosed by the gates.
The flowof electrons is not impaired, as it flows through the vertical inversion channel at
the edges of the two gates. The "stored" holes inject more electrons from the n-region of
the emitter into the n
-base, to maintain the charge carrier neutrality in this area. This
process also explains the name of the IEGT, as a greater injection of charge carriers
takes place. The same principle applies for the Trench IGBT as described in chapter
Because the aim is to stop having to use GTOs in applications, IEGTs are offered in
press pack packages (chapter
at voltage ranges of 3.3kV and4.5kV.
is aprotected trademark of Mitsubishi Electric.
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