An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily forming an electronic switch. It was developed to combine high efficiency with fast switching. It consists of four alternating layers (NPNP) that are controlled by a metal–oxide–semiconductor (MOS) gate structure. Although the structure of the IGBT is topologically similar to a thyristor with a "MOS" gate (MOS-gate thyristor), the thyristor action is completely suppressed, and only the transistor action is permitted in the entire device operation range. It is used in switching power supplies in high-power applications: variable-frequency drives (VFDs) for motor control in trains, electric cars, variable-speed refrigerators and air conditioners, as well as lamp ballasts, arc-welding machines, photovoltaic and hybrid inverters, uninterruptible power supply systems (UPS), and induction stoves. Since it is designed to turn on and off rapidly, the IGBT can synthesize complex waveforms with pulse-width modulation and low-pass filters, thus it is also used in switching amplifiers in sound systems and industrial control systems. In switching applications, modern devices feature pulse repetition rates well into the ultrasonic-range frequencies, which are at least ten times higher than audio frequencies handled by the device when used as an analog audio amplifier. As of 2010, the IGBT was the second most widely used power transistor, after the power MOSFET.
Device structure
An IGBT cell is constructed similarly to an 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. The whole structure comprises a four-layered NPNP.
History The bipolar point-contact transistor was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley. The junction version, known as the bipolar junction transistor (BJT), was invented by Shockley in 1948. Later a similar thyristor was proposed by William Shockley in 1950 and developed in 1956 by power engineers at General Electric (GE). The metal–oxide–semiconductor field-effect transistor (MOSFET) was later invented at Bell Labs between 1959 and 1960. The basic IGBT mode of operation, where a pnp transistor is driven by a MOSFET, was first proposed by K. Yamagami and Y. Akagiri of Mitsubishi Electric in the Japanese patent S47-21739, which was filed in 1968.
In 1978 J. D. Plummer and B. Scharf patented a NPNP transistor device combining MOS and bipolar capabilities for power control and switching. The development of the IGBT was characterised by an effort to completely suppress the thyristor operation, or the latch-up, in the four-layer device because the latch-up caused fatal device self-destruction. IGBTs had thus been established when the complete suppression of the latch-up of the parasitic thyristor was achieved. Later, Hans W. Becke and Carl F. Wheatley developed a similar device claiming non-latch-up. They patented the device in 1980, referring to it as "power MOSFET with an anode region" for which "no thyristor action occurs under any device operating conditions". A. Nakagawa et al. invented the device design concept of non-latch-up IGBTs in 1984. The invention is characterised by the device design setting the device saturation current below the latch-up current, which triggers the parasitic thyristor. This invention achieved complete suppression of the parasitic thyristor action for the first time, because the maximal collector current was limited by the saturation current and never exceeded the latch-up current. In the early development stage of the IGBT, research was aimed at increasing the latch-up current itself to suppress the latch-up of the parasitic thyristor. However, all these efforts failed because the IGBT could conduct an enormously large current. Successful suppression of latch-up became possible by constraining the maximal collector current to stay below the latch-up current, by controlling or reducing the saturation current of the inherent MOSFET. This was the breakthrough behind the non-latch-up IGBT, which in turn made "Becke’s device" possible. The IGBT is characterised by its ability to simultaneously handle a high voltage and a large current. The product of the voltage and the current density that the IGBT can handle reached more than 5×105 W/cm2, which far exceeded the value, 2×105 W/cm2, of existing power devices such as bipolar transistors and power MOSFETs. This is a consequence of the large safe operating area of the IGBT. The IGBT is the most rugged and the strongest power device yet developed, affording ease of use and so displacing bipolar transistors and even gate turn-off thyristors (GTOs). This excellent feature of the IGBT had suddenly emerged when the non-latch-up IGBT was established in 1984 by solving the problem of so-called "latch-up", which is the main cause of device destruction or device failure. Before that, the developed devices were very weak and were easily destroyed by "latch-up".
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