IGBT Modules - Technologies, Driver and Application (Second Edition) - page 52

less than that in a power MOSFETwith comparable current and blocking voltage. In the
latter, only themajority carriers are available. In contrast, aMOSFET can be turned on
at the gate with less energy than a bipolar transistor, which requires a relatively high
current that has tobemaintained throughout the on-period.
Combining the gate behaviour of aMOSFETwith the conduction behaviour of a bipolar
transistor produces an insulated gate bipolar transistor (IGBT)
. In terms of internal
design, the IGBT is structured much like a vertical MOSFET, except that it has an
additional high-doped p
-zone at the drain terminal which, in an IGBT, is called the
collector. If the IGBT is turned off, i.e. there is either negative voltage or 0V at the gate,
and the voltage at the emitter is more negative than at the collector, the IGBT is
operating in forward blockingmode and the pn-junction J
is blocking. The pn-junctions
and J
, on the other hand, are conductive. In order to achieve sufficient blocking
capability, then
-basemust bewide enough and exhibit sufficiently lowdoping.
Fig. 1.29
Layer sequenceandequivalent circuit diagrams of an IGBT (not to scale)
If a positive voltage (usually 15V) is applied to the gate of the IGBT, the IGBT turns on
and changes over to forward conduction mode. First, a conducting channel is formed
beneath the gate-oxide in the p-region by inversion. The channel enables an electron
current to flow from the emitter into the n
-base. This reduces the potential of the n
and opens the pn-junction J
.Minority carriers (holes) are transported from the p
to the n
-base, which exceeds the concentration of the doping level of this region by
several powers of magnitude (provided that the collector voltage is high enough). This
high concentration of holes causes the electrons to move in greater numbers from the
emitter n
-region to the n
-base, to maintain the charge neutrality. Due to the inflow of
charge carriers to the low doped and therefore relatively high-impedance n
-base, its
conductivity increases dramatically. This process is called conductancemodulation, and
sharply reduces the voltage drop across the collector-emitter path of the IGBT. As a
result, in contrast to a comparable MOSFET, there is much less conduction loss,
Theoretical work and publications that eventually lead to the contemporary IGBT commenced in the 1960s but
it was the early to mid-1980s before the first commercially mature IGBTs were launched on the market. Those
involved in the essential development of IGBTs included German engineer Hans W. Becke (1926 - 198?) and
United States engineer Carl Frank Wheatley (1927), with their patent for a "Power MOSFET with an Anode
Region" in 1982.
1...,42,43,44,45,46,47,48,49,50,51 53,54,55,56,57,58,59,60,61,62,...548
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