IGBT Modules - Technologies, Driver and Application (Second Edition) - page 365

mechanism. Based on a BARITT (Barrier Injection Transit Time) diode this mechanism
will be illustrated below.
BARITT diodes are used in microwave oscillators as an excitation component. In this
function one of the diode’s pn-junctions is biased slightly in forward direction while the
other junction is biased slightly negative. As long as the externally applied voltage is
below the critical Punch-Through voltage only a small blocking current will flow,
determined by the negatively biased pn-junction. As soon as the external voltage
reaches the Punch-Through voltage the space-charge region will extend right through
the n-region all the way to the positively biased pn-junction. At this point thermal
emission brings minority carriers (holes) from this junction into the n-region and the
current through the diode rises sharply. A negative differential resistance forms due to
the charge flow and the retarded voltage propagation across the semiconductor caused
by it. If this negative differential resistance is greater than all other positive resistances
in the resonant circuit put together then it will start to oscillate. The resonant frequency
is dependent on the transit time of the carriers in the n-region of the diode. This explains
the originof the diode’s name.
Fig. 9.5
Layermodel and I/Udiagram of aBARITTDiod
The PETTmechanism
) is similar to that of a BARITT diode. The difference is
that the space-charge region does not extend to the opposing pn-junction to discharge
carriers there. Instead, these carriers are "diverted" from the remaining n-region plasma
which hasn’t been taken over by the space-charge region andwhich is the source of the
carriers for the tail current. A current flow through the semiconductor is triggered by this
diversion of carriers into the space-charge region. Thismay cause oscillations to start if
certain conditions apply – namely that the negative differential resistance is greater than
the sum of all positive resistances in the resonant circuit. In principle the PETT
mechanism can occur in all semiconductors with a bipolar characteristic. Thus too in
IGBTs and diodes.
Aside from the p
-structur there are BARITT diodes with two Schottky contacts, which comprise of anmnm
structure. Them stands formetal contact.
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