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 (P–N–P–N) 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), electric cars, trains, variable-speed refrigerators, lamp ballasts, arc-welding machines, induction hobs, and air conditioners. 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. , 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. History The metal–oxide–semiconductor field-effect transistor (MOSFET) was invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959. 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. Following the commercialization of power MOSFETs in the 1970s, B. Jayant Baliga submitted a patent disclosure at General Electric (GE) in 1977 describing a power semiconductor device with the IGBT mode of operation, including the MOS gating of thyristors, a four-layer VMOS (V-groove MOSFET) structure, and the use of MOS-gated structures to control a four-layer semiconductor device. He began fabricating the IGBT device with the assistance of Margaret Lazeri at GE in 1978 and successfully completed the project in 1979. The results of the experiments were reported in 1979. The device structure was referred to as a "V-groove