IGBT - Insulated Gate Bipolar Transistor
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The insulated gate bipolar transistor (IGBT) is a device which combines the fast acting features and high power capability of the bipolar transistor, with the voltage control features of the MOSFET gate.
In simple terms for IGBT, the collector emitter characteristics are similar to those of the bipolar transistor but control features are those of MOSFET.
THe cross section of an IGBT is similar to that of the MOSFET, the difference being the substrate of IGBT is P-N rather than the N-N of the MOSFET.
The application of the gate voltage forms a channel for current flow, as in the case of MOSFET, which is then the base current for transistor whose path is from collector to emitter. It should be noted that although the NPN structure is invariably used for the bipolar power transistor, the PNP structure is preferred in IGBT.
Comparison of IGBT with MOSFET & BJT
The switching time for IGBT are less than of the BJT, in particular the turn ON time, typically 0.15 micro sec, being associated with the MOSFET characteristics, although the turn OFF time, typically 1 micro sec, is more rated to that of the PNP characteristic.
ON-State Collector Emitter Voltage
The ON-State collector emitter voltage is slightly higher than that of the BJT.
Voltage & Current Ratings
Maximum possible voltage & current ratings are approximately equal to those of the bipolar transistor.
Gate Circuit Requirements
The gate control is that of a MOSFET i.e. it is voltage charge controlled. The two way switch shown being two transistors. During turn-ON, the gate current is determined by the gate circuit resistance taking into account the gate charge characteristic. During the off period, a reverse bias may be necessary, depending on the nature of the load to which the IGBT is connected.