Insulated-Gate Bipolar Transistor (IGBT)

The insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device that merges characteristics of two earlier device families. From the metal-oxide-semiconductor field-effect transistor (MOSFET) it borrows a voltage-controlled insulated gate, so the control terminal draws almost no steady-state current and is easy to drive. From the bipolar junction transistor (BJT) it borrows conductivity modulation in a wide, lightly doped drift region, which gives a low on-state voltage drop even at high current. Its terminals are usually called the gate, collector and emitter.

This combination makes the IGBT well suited to applications that handle large currents at medium to high voltages. A small voltage applied between gate and emitter creates an inversion channel that turns the device on, after which injected charge carriers flood the drift region and reduce its resistance. Compared with a power MOSFET of similar rating, an IGBT typically conducts large currents with a lower forward drop, while compared with a bipolar transistor it is far simpler to switch because the gate is capacitive rather than current-driven.

IGBTs are widely used in motor drives, inverters, switched-mode power supplies, induction heating, uninterruptible power supplies, and traction systems for electric and hybrid vehicles and trains. A practical limitation is switching speed: stored minority charge produces a current “tail” during turn-off, so IGBTs generally switch more slowly than MOSFETs and are most efficient at the lower switching frequencies common in high-power converters. Devices are often packaged as modules combining several IGBTs with anti-parallel freewheeling diodes.

Frequently asked questions

How does an IGBT differ from a power MOSFET?

Both use an insulated gate, but the IGBT adds bipolar conduction in its drift region. This lowers the on-state voltage drop at high current, at the cost of slower switching due to stored charge.

When is an IGBT preferred over a MOSFET?

IGBTs are generally chosen for higher-voltage, higher-current applications switched at moderate frequencies, such as motor drives and inverters. MOSFETs are favoured at lower voltages and higher switching frequencies.

What is the current tail in an IGBT?

When the device turns off, minority carriers stored in the drift region take time to recombine, producing a residual collector current. This tail increases turn-off switching losses.