IGBT Modules - Technologies, Driver and Application (Second Edition) - page 38

Another way is an adjustment of the axial profile of the carrier lifetime to achieve the
desired switching behaviour. In this case, a recombination centre is implanted in the pn-
junctionarea. Diodes of this type are called coaxial lifetime control (CAL) diodes.
1.2.2 Mains (rectifier) diodes
Because of the lowmains frequency of 15
Hz to 60Hz, depending on the application,
diodes for rectification of the mains voltage are optimised specifically for low forward
power loss. As a consequence, their reverse recovery time t
is greater than that of fast
recovery diodes.
Such rectifier diodes are nowavailable up to several kV blocking voltage and currents of
several kA.
1.2.3 Schottkydiodes
In the Schottky
or hot carrier diode, the rectifier effect is based on the formation of a
potential barrier on a metal to semiconductor contact (Schottky contact). Compared to
diodes with a pn-junction, Schottky diodes have less "inertia" when switching over from
forward direction to reverse direction. They are therefore suited to rectification of high-
frequency alternating currents or fast switching tasks.
A Schottky contact is created between a metal and an n-doped semiconductor. If the
exertion work, i.e. releasing the energy around an electron from a solid object, in the
metal is greater than in the semiconductor, electrons will move from the semiconductor
to the metal until the two Fermi levels (chapter
are the same. Therefore, an
electron depletion zone forms on the semiconductor side and an enhancement zone on
the metal side, the electric field of which suppresses any further transfer of electrons
from the semiconductor. When an electric voltage is applied in the direction of flow
(positive terminal on the metal), electrons from the semiconductor can transfer to the
metal so that forward current flows. With the opposite polarity, the space-charge region
increases and there is only a low, temperature-dependent reverse current. TheSchottky
contact therefore has a similar rectification effect to a pn-junction, but without any
storage or inertia effect, because only the majority carriers determine its behaviour.
Schottky diodes are therefore highly suitable for high-frequency applications, for which
normal pn-diodes are somewhat limited.
However, silicon-based Schottky diodes are not usually used for blocking voltages over
200V, because their forward losses depend very much on the voltage range and the
associated thick epitaxial layer. It is not possible to reduce the ohmic proportion of this
layer by the conductancemodulation common toPIN diodes because of the lack of a p-
layer. Despite its greater "inertia", the voltage range over 200V is therefore the domain
of PINpower diodes.
Starting with the first silicon based Schottky diodes recent years have seen the
development of a new type of diode using silicon carbide (SiC). These are used in state-
of-the-art power electronic equipment, where efficiency is critical. Silicon carbide has the
followingadvantageswhen compared to silicon:
Named after the German physicist Walter Schottky (1886 - 1976). The actual rectifier effect, based on one
metal contact and one semiconductor contact, was in fact discovered by the German physicist Ferdinand Braun
(1850 - 1918).
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