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.
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.
Blocking Operation
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.
On-state Operation
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.