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| Fig.1 shows the structure of a typical n-channel
IGBT. All discussion here will be concerned with the n-channel
type but p-channel IGBT's can be considered in just the same
way. |
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| The structure is very similar to
that of a vertically diffused MOSFET featuring a double diffusion
of a p-type region and an n-type region. An inversion layer
can be formed under the gate by applying the correct voltage
to the gate contact as with a MOSFET. The main difference is
the use of a p+ substrate layer for the drain. The effect is
to change this into a bipolar device as this p-type region injects
holes into the n-type drift region. |
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| The on/off state of the device is
controlled, as in a MOSFET, by the gate voltage VG. If the voltage
applied to the gate contact, with respect to the emitter, is
less than the threshold voltage Vth then no MOSFET inversion
layer is created and the device is turned off. When this is
the case, any applied forward voltage will fall across the reversed
biased junction J2. The only current to flow will be a small
leakage current. |
| The forward breakdown voltage is
therefore determined by the breakdown voltage of this junction.
This is an important factor, particularly for power devices
where large voltages and currents are being dealt with. The
breakdown voltage of the one-sided junction is dependent on
the doping of the lower-doped side of the junction, i.e. the
n- side. This is because the lower doping results in a wider
depletion region and thus a lower maximum electric field in
the depletion region. It is for this reason that the n- drift
region is doped much lighter than the p-type body region. The
device that is being modelled is designed to have a breakdown
voltage of 600V. |
| The n+ buffer layer is often present
to prevent the depletion region of junction J2 from extending
right to the p bipolar collector. The inclusion of this layer
however drastically reduces the reverse blocking capability
of the device as this is dependent on the breakdown voltage
of junction J3, which is reverse biased under reverse voltage
conditions. The benefit of this buffer layer is that it allows
the thickness of the drift region to be reduced, thus reducing
on-state losses. |
| The turning on of the device is achieved
by increasing the gate voltage VG so that it is greater than
the threshold voltage Vth. This results in an inversion layer
forming under the gate which provides a channel linking the
source to the drift region of the device. Electrons are then
injected from the source into the drift region while at the
same time junction J3, which is forward biased, injects holes
into the n- doped drift region (Fig.2). |
| This injection causes conductivity
modulation of the drift region where both the electron and hole
densities are several orders of magnitude higher than the original
n- doping. It is this conductivity modulation which gives the
IGBT its low on-state voltage because of the reduced resistance
of the drift region. Some of the injected holes will recombine
in the drift region, while others will cross the region via
drift and diffusion and will reach the junction with the p-type
region where they will be collected. The operation of the IGBT
can therefore be considered like a wide-base pnp transistor
whose base drive current is supplied by the MOSFET current through
the channel. A simple equivalent circuit is therefore as shown
in Fig.3(a) |
| Fig.3(b) shows a more complete equivalent
circuit which includes the parasitic npn transistor formed by
the n+-type MOSFET source, the p-type body region and the n--type
drift region. Also shown is the lateral resistance of the p-type
region. If the current flowing through this resistance is high
enough it will produce a voltage drop that will forward bias
the junction with the n+ region turning on the parasitic transistor
which forms part of a parasitic thyristor. Once this happens
there is a high injection of electrons from the n+ region into
the p region and all gate control is lost. This is known as
latch up and usually leads to device destruction. |
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