IGBT | ||||
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Structural description The n channel IGBT has one vertical structure similar to the PowerMos. As it is shown at in Fig.1 it is possible to distinguish various type of layers and various doping. The diversity of the structure of the IGBT regarding the structure of the PowerMos is in the presence of a thin layer N+, much doped, called buffer_layer or HDB (High Doped Base). The wide N- region, called region of drift or LDB (Low Doped Base), together to body (P) and the substrate (P+) forms a P N- P+ structure that can be considered like a PNP BJT.
Fig. 1 cross-section of the IGBT. The region of drift, for the fact
that is weakly doped, allows the IGBT sustain high voltages while
the N+ region has the double function to control the gain of the
transistor and to limit the emptying of the base collector junctions avoiding that this catches up the emitter. The first generation devices
did not introduce the N+ zone and are called symmetrical IGBT or (NPT-IGBT). The more recent
devices introduce the structure of Fig. 1 and are said asymmetric
IGBT or (PT-IGBT). In macroscopic terms the IGBT can
be seen like the connection of a PowerMos and a PNP BJT whose base is
connected to the drain of the MOS (Fig. 2a).
Fig.2 a) Circuit equivalent of the IGBT; b)
Symbol of the IGBT. The IGBT, whose symbol is brought back in Fig. 2b, has therefore the input characteristics of the PowerMos and the output characteristics of a BJT. Externally the IGBT is accessible by means of three pins called the collector, gate , emitter that makes head respective the emitter of the BJT, to the gate of the MOS and the common node between the MOS source and the BJT collector. |
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