The insulated-gate bipolar transistor is a three-terminal power semiconductor device
primarily used as an electronic switch which, as it was developed, came to combine high
efficiency and fast switching. It switches electric power in many modern appliances:
variable-frequency drives, electric cars, trains, variable speed refrigerators, lamp
ballasts, air-conditioners and even stereo systems with switching amplifiers. Since it
is designed to turn on and off rapidly, amplifiers that use it often synthesize complex waveforms
with pulse width modulation and low-pass filters. In switching applications modern devices boast
pulse repetition rates well into the ultrasonic range—frequencies which are at least ten
times the highest audio frequency handled by the device when used as an analog audio
amplifier. The IGBT combines the simple gate-drive characteristics
of MOSFETs with the high-current and low-saturation-voltage capability of bipolar transistors. The IGBT
combines an isolated gate FET for the control input, and a bipolar power transistor as a
switch, in a single device. The IGBT is used in medium- to high-power applications like
switched-mode power supplies, traction motor control and induction heating. Large IGBT
modules typically consist of many devices in parallel and can have very high current
handling capabilities in the order of hundreds of amperes with blocking voltages of 6000
V, equating to hundreds of kilowatts. The first-generation IGBTs of the 1980s and
early 1990s were prone to failure through such modes as latchup and secondary breakdown.
Second-generation devices were much improved, and the current third-generation ones are
even better, with speed rivaling MOSFETs, and excellent ruggedness and tolerance of
overloads. The extremely high pulse ratings of second-
and third-generation devices also make them useful for generating large power pulses in
areas including particle and plasma physics, where they are starting to supersede older
devices such as thyratrons and triggered spark gaps.
Their high pulse ratings, and low prices on the surplus market, also make them attractive
to the high-voltage hobbyist for controlling large amounts of power to drive devices such
as solid-state Tesla coils and coilguns. Affordable, reliable IGBTs are important for
electric vehicles and hybrid cars.
History
The IGBT is a semiconductor device with four alternating layers that are controlled by
a metal-oxide-semiconductor gate structure without regenerative action. This mode of
operation was first proposed by Yamagami in his Japanese patent S47-21739, which was filed
in 1968. This mode of operation was first experimentally discovered by B. J. Baliga.
The device structure was referred to as a ‘V-groove MOSFET device with the drain region
replaced by a p-type Anode Region’ in this paper and subsequently as 'the insulated-gate
rectifier', the insulated-gate transistor, the conductivity-modulated field-effect transistor
and "bipolar-mode MOSFET". Plummer found the same IGBT mode of operation
in the four layer device and he first filed a patent application for the device structure
in 1978. USP No.4199774 was issued in 1980 and B1 Re33209 was reissued in 1995 for the
IGBT mode operation in the four layer device. Hans W. Becke and Carl F. Wheatley invented
a similar device for which they filed a patent application in 1980, and which they referred
to as "power MOSFET with an anode region". This patent has been called "the seminal patent
of the insulated gate bipolar transistor." The patent claimed "no thyristor action occurs
under any device operating conditions." This substantially means that the device exhibits
non-latch-up IGBT operation over the entire device operation range.
Practical devices capable of operating over an extended current range were first reported
by Baliga et al. in 1982. A similar paper was also submitted by J.P. Russel et al. to
IEEE Electron Device Letter in 1982. The applications for the device were initially regarded by
the power electronics community to be severely restricted by its slow switching speed and
latch-up of the parasitic thyristor structure inherent within the device. However, it was
demonstrated by Baliga and also by A.M. Goodman et al. in 1983 that the switching speed could
be adjusted over a broad range by using electron irradiation. This was followed by demonstration
of operation of the device at elevated temperatures by Baliga in 1985. Successful efforts to suppress
the latch-up of the parasitic thyristor and the scaling of the voltage rating of the devices
at GE allowed the introduction of commercial devices in 1983, which could be utilized for
a wide variety of applications. Complete suppression of the parasitic thyristor
action and the resultant non-latch-up IGBT operation for the entire device operation
range was achieved by A. Nakagawa et al. in 1984. The non-latch-up design concept was
filed for US patents. To test the lack of latchup, the prototype 1200V IGBTs were directly
connected without any loads across a 600V constant voltage source and were switched
on for 25 microseconds. The entire 600V was dropped across the device and a large short
circuit current flowed. The devices successfully withstood this severe condition. This was
the first demonstration of so-called "short-circuit-withstanding-capability" in IGBTs. Non-latch-up IGBT operation was
ensured, for the first time, for the entire device operation range. In this sense, the
non-latch-up IGBT proposed by Hans W. Becke and Carl F. Wheatley was realized by A. Nakagawa
et al. in 1984. Products of non-latch-up IGBTs were first commercialized by Toshiba in 1985.
Once the non-latch-up capability was achieved in IGBTs, it was found that IGBTs exhibited
very rugged and a very large safe operating area. It was demonstrated that the product
of the operating current density and the collector voltage exceeded the theoretical limit of
bipolar transistors, 2x105W/cm2, and reached 5x105W/cm2.
The insulating material is typically made of solid polymers which have issues with degradation.
There are developments that use an ion gel to improve manufacturing and reduce the voltage
required. Device structure
An IGBT cell is constructed similarly to a n-channel vertical construction power MOSFET
except the n+ drain is replaced with a p+ collector layer, thus forming a vertical PNP
bipolar junction transistor. This additional p+ region creates a cascade
connection of a PNP bipolar junction transistor with the surface n-channel MOSFET.
Comparison with power MOSFETs An IGBT features a significantly lower forward
voltage drop compared to a conventional MOSFET in higher blocking voltage rated devices.
As the blocking voltage rating of both MOSFET and IGBT devices increases, the depth of the
n- drift region must increase and the doping must decrease, resulting in roughly square
relationship decrease in forward conduction vs. blocking voltage capability of the device.
By injecting minority carriers from the collector p+ region into the n- drift region during
forward conduction, the resistance of the n- drift region is considerably reduced. However,
this resultant reduction in on-state forward voltage comes with several penalties:
The additional PN junction blocks reverse current flow. This means that unlike a MOSFET,
IGBTs cannot conduct in the reverse direction. In bridge circuits, where reverse current
flow is needed, an additional diode is placed in parallel with the IGBT to conduct current
in the opposite direction. The penalty isn't overly severe because at higher voltages,
where IGBT usage dominates, discrete diodes are of significantly higher performance than
the body diode of a MOSFET. The reverse bias rating of the N-drift region
to collector P+ diode is usually only of tens of volts, so if the circuit application applies
a reverse voltage to the IGBT, an additional series diode must be used.
The minority carriers injected into the N-drift region take time to enter and exit or recombine
at turn on and turn off. This results in longer switching times, and hence higher switching
loss compared to a power MOSFET. The on-state forward voltage drop in IGBTs
behaves very differently from power MOSFETS. The MOSFET voltage drop can be modeled as
a resistance, with the voltage drop proportional to current. By contrast, the IGBT has a diode-like
voltage drop increasing only with the log of the current. Additionally, MOSFET resistance
is typically lower for smaller blocking voltages, so the choice between IGBTs and power MOSFETS
will depend on both the blocking voltage and current involved in a particular application.
In general, high voltage, high current and low switching frequencies favor IGBTs while
low voltage, low current and high switching frequencies are the domain of the MOSFET.
IGBT models Rather than using a device physics-based model,
SPICE simulates IGBTs using Macromodels, a method that combines an ensemble of components
like FETs and BJTs in a Darlington configuration. An alternative physics-based model is the
Hefner model, introduced by Allen Hefner of the NIST. It is a fairly complex model that
has shown very good results. Hefner's model is described in a 1988 paper and was later
extended to a thermo-electrical model and a version using SABER.
Usage See also
Bootstrapping FGMOS
Power electronics Power MOSFET
Solar inverter Variable-frequency drive
References
Further reading Dr. Ulrich Nicolai, Dr. Tobias Reimann, Prof.
Jürgen Petzoldt, Josef Lutz: Application Manual IGBT and MOSFET Power Modules, 1. Edition,
ISLE Verlag, 1998, ISBN 3-932633-24-5 PDF-Version Wintrich, Arendt; Nicolai, Ulrich; Tursky,
Werner; Reimann, Tobias. [PDF-Version Application Manual 2011]. Nuremberg: Semikron. ISBN 978-3-938843-66-6.
External links Device physics information from the University
of Glasgow Spice model for IGBT
IGBT driver calculation