IGBT Modules - Technologies, Driver and Application (Second Edition) - page 41

Fig. 1.18
Voltage ranges of theZener and avalanche effects
The Zener effect in highly doped pn-junctions that are sufficiently biased in the reverse
direction describes the conversion of electrons from the valence band of the p-doped
region to the conduction band of the n-doped region via tunnelling. This creates a strong
current in the reverse direction (Zener current) . The Zener effect is caused by the bias
voltage or the considerable field strength it causes in the pn-junction, which cause the
energy bands in the p-doped and n-doped region are displaced against each other in
such a way that unoccupied states in the conduction band have the same energy as
occupied states in the valence band. This makes it possible for electrons tomove from
the valence band to the conduction band with certain likelihood without absorbing
energy. Because they flow through the no-go zone of the bandgap
without any
additional external energy, this is described as a quantum mechanical tunnelling
through the forbidden energy zone.
The avalanche effect (chapte
describes the snowball-likemultiplication of charge
carriers in a semiconductor within the barrier of an existing pn-junctionwhen sufficiently
high blocking voltage is applied. This accelerates the charge carriers there to such an
extent that their kinetic energy is sufficient to bring secondary electrons from the
valence band into the conduction band so that the reverse current suddenly increases
sharply and causes anavalancheof carriers (chapte
The forward region of I/U diagrams for Z-diodes is identical to that of other low power
diodes. Depending on the doping and the gauge of the drift zone of the Z-diode,
however, various well-defined breakdown voltages U
can be activated with regard to
the reverse area.
Fig. 1.19
Circuit symbols and I/Udiagram for Z-diodes
1...,31,32,33,34,35,36,37,38,39,40 42,43,44,45,46,47,48,49,50,51,...548
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