1、What is an IGBT?

IGBT, or Insulated Gate Bipolar Transistor, is a composite and fully-controlled voltage-driven power semiconductor device composed of Bipolar Junction Transistor (BJT) and insulated-gate Field Effect Transistor (MOSFET). It combines the high input impedance of a MOSFET and the low conduction voltage drop of a power transistor (GTR). GTRs have low saturation voltage and high current density but require a large driving current (since Vbe = 0.7V, and Ic can be very large depending on the PN junction material and thickness). MOSFETs have small driving power and fast switching speeds but suffer from higher conduction voltage drops and lower current density (due to the Rds in MOSFET, high Ids will lead to high Vds ). IGBT integrates the advantages of both the two device, offering low driving power and saturation voltage. This makes it particularly suitable for the use in conversion systems with DC voltages of 600V and above, such as AC motors, inverters, switching power supplies, lighting circuits and traction drives. The primary function of an IGBT is to convert high-voltage DC to AC and enable frequency conversion.

2、Basic Structure and Working Principle of IGBT

As shown in image 1, an IGBT is a composite device composed of a MOSFET and a PNP transistor. It can also be regarded as a combination of a VDMOS and a PN diode. Image 2 illustrates the equivalent circuit of an IGBT.

       (Image 1: Basic Structure of an IGBT Cell)

          (Image 2: Equivalent Circuit of an IGBT Device)

(1) Static Characteristics of IGBT

Conventional IGBTs have forward blocking capability provided by the collector junction of the PNP transistor, but their reverse voltage tolerance is limited to a few tens of volts due to the lack of terminal or surface structures at the emitter junction. When in on-state condition, apart from a diode threshold voltage (approximately 0.7V), the output characteristics of an IGBT are identical to those of a VDMOS. Image 3 shows the curve of the forward and reverse DC characteristics of an IGBT device.

Key Static Parameters of IGBT:

• Blocking Voltage (V(BR)CES): The voltage tolerance of the device in the forward blocking state.
• On-state Voltage Drop (VCE(on)): The voltage drop of the device in the on-statecondition.
• Threshold Voltage (VGEth): The gate voltage of device required to applied from the blocking state to the on-state.

(Image 3: Forward and Reverse DC Characteristics of an IGBT Device)

(2) Switching Characteristics of IGBT

The switching mechanism of an IGBT is identical to that of a VDMOS, with the MOS gate controlling its turn-on and turn-off states. The difference is that an IGBT includes an additional PN junction at the drain, which involves minority carrier holes during conduction. This phenomenon, known as the conductivity modulation effect, results in a lower on-state voltage drop of IGBTs compared to VDMOS at the same voltage rating. However, the presence of holes in the drift region extends the turn-off time of the IGBT compared to VDMOS multi-carrier devices.

Key Switching Parameters of IGBT:

• Turn-on Time (td(on) + tr): The time required for the device to switch from the blocking state to the on-state.

• Turn-off Time (td(off) + tf): The time required for the device to switch from theon-state to the blocking state.

• Turn-on Energy (Eon): Energy loss during the turn-on state.

• Turn-off Energy (Eoff): Energy loss during the turn-off state.

        (Image 4: Switching Characteristics of an IGBT Device)