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Insulated Gate Bipolar Transistor

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The Insulated Gate Bipolar Transistor, IGBT

The Insulated Gate Bipolar Transistor also called an IGBT for short, is a cross between a conventional Bipolar Junction Transistor, or “BJT” and a Field Effect Transistor, or “FET”. The IGBT transistor takes the best parts of these two types of transistors and combines them together to produce another type of transistor switching device that is capable of handling large collector-emitter currents with virtually zero gate current drive.

Typical IGBT
igbt transistor

The insulated gate bipolar transistor, (IGBT) uses the insulated gate (hence the first part of its name) technology of the MOSFET with the output performance characteristics of a conventional bipolar transistor, (hence the second part of its name). The result of this combination is that the IGBT transistor has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a MOSFET.

The advantage gained by the IGBT is that it offers greater power gain than the bipolar type together with the higher voltage operation and lower input losses of the MOSFET. In effect it is an FET integrated with a bipolar transistor in a form of Darlington configuration as shown.

Insulated Gate Bipolar Transistor

insulated gate bipolar transistor

We can see that the insulated gate bipolar transistor is a three terminal, transconductance device with two of the terminals (C-E) associated with a conductance path and the third terminal (G) associated with its control. The amount of amplification achieved by the insulated gate bipolar transistor is a ratio between its output signal and its input signal.

For a conventional bipolar junction transistor, the amount of gain is approximately equal to the ratio of the output current to the input current, called Beta. For a metal oxide semiconductor field effect transistor or MOSFET, there is no input current as the gate is isolated from the main current carrying channel. Therefore, an FET’s gain is equal to the ratio of output current change to input voltage change, making it a transconductance device and this is also true of the IGBT.

The Insulated Gate Bipolar Transistor can be used in small signal amplifier circuits in much the same way as the BJT or MOSFET type transistors. But as the IGBT combines the low conduction loss of a BJT with the high switching speed of a power MOSFET an optimal solid state switch exists which is ideal for use in power electronics applications.

When used as static controlled switch, the insulated gate bipolar transistor has voltage and current ratings similar to that of the bipolar transistor. However, the presence of an isolated gate in an IGBT makes it a lot simpler to drive than the BJT. The IGBT is turned “ON” or “OFF” by activating and deactivating its Gate terminal. A constant positive voltage input signal across the Gate and the Emitter will keep the device in its “ON” state, while removal of the input signal will cause it to turn “OFF” in much the same way as a bipolar transistor or MOSFET.

In other words, because the IGBT is a voltage-controlled device, it only requires a small voltage on the Gate to maintain conduction through the device unlike BJT’s which require that the Base current is continuously supplied in a sufficient enough quantity to maintain saturation. Also the IGBT is a unidirectional device, meaning it can only switch current in the “forward direction”, that is from Collector to Emitter unlike MOSFET’s which have bi-directional current switching capabilities (controlled in the forward direction and uncontrolled in the reverse direction).

The principal of operation and Gate drive circuits for the insulated gate bipolar transistor are very similar to that of the N-channel power MOSFET. The basic difference is that the resistance offered by the main conducting channel when current flows through the device in its “ON” state is very much smaller in the IGBT. Because of this, the current ratings are much higher when compared with an equivalent power MOSFET.

The main advantages of using the Insulated Gate Bipolar Transistor over other types of transistor devices are its high voltage capability, low ON-resistance, ease of drive, relatively fast switching speeds and combined with zero gate drive current makes it a good choice for moderate speed, high voltage applications such as in pulse-width modulated (PWM), variable speed control, switch-mode power supplies or solar powered DC-AC inverter and frequency converter applications operating in the hundreds of kilohertz range.


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