What is an IGBT？

IGBT or Insulated Gate Bipolar Transistor is a voltage-controlled power electronic switching device that combines the best characteristics of both BJTs and MOSFETs. IGBTs are represented symbolically as seen in the figure below.

An IGBT is a voltage-controlled device that can be turned on/off by regulating the voltage applied across the gate and emitter terminals. To turn it on, a voltage greater than the threshold voltage needs to be applied between the gate and the emitter. To turn it off, a negative voltage is applied between the gate and emitter terminals. The value of threshold voltage depends on the datasheet of an individual component. The collector-emitter provides a conductance path to the current. This phenomenon can be explained by the figure below:

A voltage greater than the threshold voltage is applied across the gate terminal to turn the IGBT on. As can be seen from the figure above, a voltage (VGE) is applied to the gate terminal, as a result, the Gate Current (IG) increases. The Gate-Emitter voltage, VGE, also increases the Collector Current (IC) which in turn increases the collector-emmiter voltage, VCE. The VCE represents a collector-emitter voltage drop in the ON state and is used to calculate the power dissipation loss of the IGBT using the equation:

VCE(on state) = IC. RCE(on state)

IGBTs are ideal for low to medium power applications such as traction inverters for HEV/EV, auxiliary DC/AC converters, switched-mode power supplies, refrigerators, industrial motors, automotive main motor controllers to improve their efficiency traction motor control, induction heating, and power train systems requiring fast switching.

The input and output characteristics of an IGBT are shown below:

Input Characteristics: IC (Collector current) vs VG (Gate voltage)

The IGBT turns on when the gate voltage crosses the threshold voltage i.e., current conduction starts between the collector and emitter terminals.

Output Characteristics: IC (Collector current) vs VCE (Collector to emitter voltage)

The output characteristics are divided into four regions –

• Cut-off region   - When the gate to emitter voltage,   VGE = 0, the device is in an OFF state and there is no flow of current between the collector and emitter. When VGE becomes > 0 but is less than the threshold voltage, a small leakage current is seen but the device is still not in the ON state.

• Linerar region – The collector current Ic should stay in this region. If the operating point at a given gate-emitter voltage goes above the linear region, any further increase in collector current, Ic, results in a significant rise in collector-emitter voltage and a consequent rise in conduction loss and possible device destruction.  

• Active region - When VGE crosses the threshold voltage, the device turns on and is said to be in an active region. The current flowing through the device depends on the collector-emitter voltage VCE. The approximate relationship between VCE and IC can be represented by this linear equation:

VCE(on state) = IC. RCE(on state)

• Breakdown region - There is a maximum voltage in forward conduction mode beyond which collector-emitter breakdown occurs and gate losses control of collector current. This results in damage to the device.

Key features of IGBT:

• Low on-state voltage as compared to the BJT (bipolar junction transistor).

• Lower switching losses.

• Lower conduction losses.

• Ease of gate drive.

• Peak current capability.

• Ruggedness.

• Faster switching than BJT.

The equivalent circuit representing an IGBT looks effectively like a combination of an N-channel MOSFET and   Bipolar transistor,

IGBTs inherit the input characteristics of MOSFETs - high input impedance (provides insulation against leakage currents) and faster switching parameters. It also inherits output characteristics of BJT- high output current rating.

IGBTs offer higher power gain and lower switching losses as compared to BJTs. They have higher switching speeds and offer significantly lower I2R losses in their bipolar output as compared to a standard BJT.

When compared to MOSFETs, IGBTs can handle higher voltage and power applications but they provide lower switching speed. Another advantage of using an IGBT over a MOSFET is that it can handle higher current values while drawing negligible gate-drive current. In theory, IGBTs can be rated up to 100s of amperes, 10 kV, and up to 50 kHz of switching frequency. An IGBT is ideally suited for high-power, medium-speed applications whereas a MOSFET is preferred for high-speed switching applications with medium-power requirements.

The differences between an IGBT, BJT, and MOSFET are summarised in the table below: