IGBT Modules - Technologies, Driver and Application (Second Edition) - page 43

1.4 Bipolar junction and field effect transistors
are active electronic components that typically have three external
In power electronics, they are used inmost applications as switches, i.e. in
a blocking or conducting state. Ideally, the switching length and switching losses
between the two states shouldbe infinitesimal.
1.4.1 Bipolar junction transistors (BJTs)
Bipolar junction transistors commonly used in power electronics applications are
distinctive in that they have four vertical doping zoneswith two pn-junctions in their layer
shows the two variants of bipolar transistors: pnp and npn. Besides
the vertical structure there are also bipolar transistors with a lateral structure, but those
will not be covered in detail here.
Each layer of a BJT has a control terminal accessible externally. The highly doped n
layer of thenpn-transistor forms the emitter terminal, followedby the p-doped base zone
and the collector, which is executed by two different n-doped layers.
The characteristics of npn-transistors are explained in detail below. The results apply in
the same way to pnp-transistors, except that the polarity and doping are the other way
Fig. 1.21
Layer sequence, circuit symbols and output characteristics of bipolar
transistors (not to scale)
With positive voltage U
at the collector-emitter terminals, but without external positive
voltage at the base terminal (U
= 0V), the transistor is in a blocked state, as the layer
sequence treats npn as two diodes connected against each other. The pn-junction is
Transistor is an invented word, created by combining the words 'transfer' and 'resistor'. The transistor effect
was officially discovered in 1947 by the United States physicists John Bardeen (1908 - 1991), Walter Houser
Brattain (1902 - 1987) andWilliamShockley (1910 - 1989). TheGerman physicistsHeinrichWelker (1912 - 1981)
and Herbert Franz Mataré (1912) were working independently from those physicists on the same topic at the
same time.
There are exceptions, including the phototransistor, in which the base terminal is replaced by a light-sensitive
base region.
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