IGBT

An IGBT is a three-terminal semiconductor switch that combines the high input impedance of a metal-oxide-semiconductor field-effect transistor (MOSFET) with the low saturation voltage and high current capacity of a bipolar junction transistor (BJT). It sits between the gate terminal and the collector-emitter path, allowing a small control signal at the gate to switch large currents, typically ranging from tens of amps to several hundred amps, with switching frequencies from 1 kHz to 20 kHz or higher depending on the device rating.

The structure stacks a MOSFET on top of a BJT, so the gate voltage controls whether the device conducts or blocks. When the gate voltage exceeds the threshold (typically 3 to 15 volts), charge carriers flood through the drift region and the device enters saturation, dropping a voltage of 0.5 to 2 volts across the collector-emitter terminals. This low on-state voltage means less heat dissipation than older thyristors or power MOSFETs carrying the same current. The insulated gate gives the device its name and its key advantage: it draws almost no steady-state gate current, so the drive circuit can be simple and efficient.

IGBTs dominate variable-frequency drives (VFDs) for motors, switched-mode power supplies (SMPS) in industrial equipment, uninterruptible power supplies (UPS), welding inverters, and high-power photovoltaic inverters. A typical 3-phase VFD uses six IGBTs arranged in a bridge configuration, switching on and off hundreds of thousands of times per second to synthesize an AC output from a DC bus. Modern IGBTs are rated from 600 volts up to 6500 volts for medium-voltage applications, and manufacturers specify maximum repetitive peak current and junction temperature limits, typically 150 degrees Celsius, beyond which reliability degrades rapidly.

Failure modes and thermal stress

IGBTs fail when the junction temperature exceeds design limits, when gate-emitter overvoltage spikes damage the oxide layer, or when parasitic oscillations at turn-off create voltage spikes exceeding the rated blocking voltage. In high-current switching applications, paralleling multiple IGBTs can share the load, but requires careful design to balance gate drive signals and collector currents. Short-circuit protection is critical: if the load suddenly shorts, the collector current can exceed rated maximum within microseconds, generating destructive power dissipation unless the gate drive circuit detects the fault and removes the gate signal.

The performance advantage of IGBTs over older power MOSFETs and thyristors lies in the sweet spot between switching speed and conduction loss. MOSFETs switch faster but conduct at higher voltage drop at high currents. Thyristors and gate-turn-off (GTO) thyristors conduct with lower voltage drop but need complex commutation circuits. IGBTs bridge that gap, which is why they have become the standard switching device in inverters and drives across nearly all industrial power conversion equipment built in the past two decades.