About IGBTs

About IGBTs

“IGBT” is an acronym of Insulated Gate Bipolar Transistor. As a hybrid of MOSFETs and bipolar transistors, they serve as power transistors that combine features of both. IGBTs are of two types, N-channel and P-channel; here we used in our example N-channel IGBTs, which are currently the mainstream.

Below are shown the circuit diagram symbol of an N-channel IGBT and the equivalent circuit of the device. Some equivalent circuits provide more detail, but here a comparatively simple circuit is shown to aid understanding. The actual device is somewhat more complex, including the structure. The structure and other aspects will be described below in greater detail.

An IGBT has three pins: the gate, the collector, and the emitter. The gate is the same as in a MOSFET; the collector and emitter are like those in a bipolar transistor. Like MOSFETs, an IGBT is a voltage-controlled device: when a positive voltage, relative to the emitter, is applied to the gate of an N-channel IGBT, there is conduction between the collector and the emitter, and a collector current flows. The operation and method of driving a IGBT will be explained separately.

As stated above, an IGBT is a transistor that combines desirable aspects of MOSFETs and bipolar transistors. Because the gate of a MOSFET is insulated, it has the advantages of a high input impedance and relatively fast switching, but with the drawback that the on-resistance is higher with higher withstand voltage ratings. A bipolar transistor has a low on-resistance even with higher withstand voltage ratings, but drawbacks include a low input impedance and slow switching speeds. By supplementing these deficiencies, an IGBT achieves high input impedance, fast switching*, and low on-resistance even with higher withstand voltage ratings.
*Switching is slower than for MOSFETs, but faster than that of bipolar transistors.

Power devices such as IGBTs and MOSFETs are used selectively according to the application usage conditions and requirements, choosing the right device for the right role, as for example when IGBTs are used for high-voltage applications while MOSFETs are employed at lower voltages. The range of application of IGBTs and applications of same will be discussed in future.