IGBT Modules - Technologies, Driver and Application (Second Edition) - page 531

The larger the volume of a semiconductor, the greater is the likelihood of
partly summarises these dependencies. The probability of failure is quoted in
Essentially neutrons and protons of the secondary radiation are responsible for the
spontaneous failure of power semiconductors. If a semiconductor is in the blocked state,
such a collision with the existing silicon in the semiconductor can produce a plasma of
charged particles. Since the component is in the blocked state, a space-charge region
has formed inside. This space-charge region separates the carriers of the plasma. If the
electric field strength of the space-charge region is large enough to allow an
acceleration of the carriers, leading to impact ionization with other silicon nuclei, then
more and more charge carriers are generated. This process occurs within a very short
time (in the order of several hundred picoseconds), and is limited to a small volume.
Local areas of high currents are created, which lead to the destruction of the
Fig. 14.33
Dependence of the probability of the failure (FIT rate) as a function of the
blocking voltage the altitude h in which the component is operated and the junction
To reduce the probability of failure of a component based on the cosmic radiation,
therefore, several measures can be taken. The manufacturer can set the internal field
profile in the blocked state so that in nominal operation the electric field is still
sufficiently low to inhibit impact ionization. The edge structure too plays an important
role, especially since it is there where regions of high field strengths appear. On the
user's side, when designing the application, the next higher voltage class thanwould be
required can be used for adevice, for example.
The definition of the blocking voltage capability is determined while considering the FIT
rate. It should be noted that this FIT rate refers to the stationary blocking operation with
a DC voltage, i.e. the component is not turned on and off as in normal operation. If the
components achieve a FIT rate of 100 in the test, then the corresponding DC voltage is
the blocking voltage U
quoted in the datasheet. For IGBT modules with a nominal
voltage rating of 3.3kV and above, however, the FIT rate of 100 is no longer referenced
to the nominal value, but to an especially designated DC voltage value quoted in the
datasheet asU
as i
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