The IGBT has a threshold voltage of
around 0.7V; a voltage drop lower than this value is not possible. The
“resistive part” of the output characteristics of an IGBT is very low, and so
it can conduct large currents with a low voltage drop. It is thus most suitable
for use at high current densities. An IGBT can be simply modeled as a
pnp-transistor driven by a MOSFET. The disadvantage of this structure is the
turn off. If a pnp transistor is to be turned off quickly, a positive base
current must be supplied, to force the carriers in the base to recombine and
stop the device conducting. In the IGBT, the base of the pnp stage cannot be
accessed directly, and so this current cannot be delivered at turn off, meaning
that the device continues to conduct while the carriers recombine
"naturally". During this time, a current tail appears.
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