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 pnp-transistor portion, apnp,
increases and a greater portion of the injected holes into the drift
region will be collected at the junction of p-base and nÿ-drift
regions.
Therefore, the magnitude of the lateral
hole current increases, which increases the lateral voltage drop. As
a result the parasitic thyristor will latch up even if the on-state
current is less than the static latch-up
value.
Reducing the gain of the npn- or
pnp-transistors can prevent the parasitic thyristor latch-up. A
reduction in the gain of the pnp-transistor increases the IGBT
on-state voltage drop. Therefore, in order to prevent the parasitic
thyristor latch up it is better to reduce the gain of the
npn-transistor component of IGBT.
Reduction of carrier lifetime, use of
buffer layer, and use of deep p‡-diffusion improve the latch-up
immunity of IGBT. However, inadequate extension of the p‡-region may
fail to prevent the device from latch-up.
Also, care should be taken that the
p‡-diffusion does not extend into the MOS channel because this
causes an increase in the MOS threshold voltage.
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