IGBT Modules - Technologies, Driver and Application (Second Edition) - page 40

role of this layer, which is known as a fieldstop, is to reduce the electrical field during the
blocking state. This allows the epitaxial layer to be appliedmore thinly, with a resulting
reduction in forward loss at the same dielectric strength. This principle is commonly
found not only in Schottky diodes but also in power PIN diodes as well as IGBTs
Fig. 1.17
Variants of SiCSchottky diodes and circuit symbols (not to scale)
1.2.4 Z-diodes and avalanchediodes
Diodes operated in reverse direction are called Z-diodes, in which the Zener or
avalanche effect is exploited during normal operation. The distinctive feature of these
diodes is their steep increase in current when a certain value of voltage in reverse
direction (the Z-voltage U
) is exceeded. The constancy of the Z-voltage over a wide
current range is applied in stabilisation circuits to stabilise the voltage, and so to achieve
a fixed reference voltage. To prevent the strong current causing destruction, each Z-
diode has to be operated with limited current, which is usually achieved using a series
In silicon diodes, the Zener effect occurs approximately between 2V and 6.5V. The
avalanche effect occurs from approximately 4.5V, so that both breakdownmechanisms
occur simultaneously between 4.5V and 6.5
. Components that can be described as
Z-diodes are for example those of the BZX84 series, which are offered for the voltage
range from 2.4V to 75V. Z-diodes with extremely fast response times and a high pulse
power handling capacity over a short period are called transient voltage suppressor
(TVS) diodes.While Z-diodes aremostly used for voltage stabilisation, themain field of
application for TVS diodes is in limiting power surges, such as in clamping circuits for
IGBTdriver stages (chapter
The name Zener diodes was previously used for all Z-diodes. This was changed at the instigation of United
States physicist Clarence Melvin Zener (1905 - 1993) because in most cases, i.e. at greater Z-voltages, the
avalanche effect is thedetermining factor, while theZener effect can be observed only up to a few volts.
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