A portion of the minority carriers injected into the drift region
from the collector of an IGBT flows directly to the emitter
terminal. The negative charge of electrons in the inversion
layer attracts the majority of holes and generates the lateral
component of hole current through the p-type body layer as
shown in Fig. 7.10. This lateral current flow develops a voltage
drop across the spreading resistance of the p-base region,
which forward-biases the base-emitter junction of the npnparasitic
BJT. By designing a small spreading resistance, the
voltage drop is lower than the built-in potential and therefore
the parasitic thyristor between the p.-collector region, nÿ-
drift region, p-base region, and n.-emitter does not latch up.
Larger values of on-state current density produce a larger
voltage drop, which causes injection of electrons from the
emitter region into the p-base region and hence turn-on of the
npn-transistor. When this occurs the pnp-transistor will turn
on, and therefore the parasitic thyristor will latch up and the
gate loses control over the collector current.
Under dynamic turn-off conditions the magnitude of the
lateral hole current flow increases and latch-up can occur at
lower on-state currents compared to the static condition. The
parasitic thyristor latches up when the sum of the current
gains of the npn- and pnp-transistors exceeds one. When the
gate voltage is removed from IGBT with a clamped inductive
load, its MOSFET component turns off and reduces the
MOSFET current to zero very rapidly. As a result the drainsource
voltage rises rapidly and is supported by the junction
between the nÿ-drift region and the p-base region. The drift
region has a lower doping and therefore the depletion layer
extends more in the drift region. Hence, the current gain of the