What is the difference between MOSFETs and IGBTs?

MOSFETs and IGBTs are voltage-driven power semiconductor devices designed for different application ranges. In general, MOSFETs are used in low- to medium-voltage applications requiring high-speed switching, while IGBTs are widely used in high-voltage and high-current applications. These differences originate from their device structures and the way charge carriers behave during conduction. This FAQ compares the structure, operating principles, conduction characteristics, and switching characteristics of MOSFETs and IGBTs, and explains how to choose the appropriate device for a given application.

Structural Differences Between MOSFETs and IGBTs
Both MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors) are voltage-driven power devices in which the current is controlled by an electrically insulated gate electrode. Both devices adopt a vertical structure in which current flows from the top surface to the backside of the chip, and the drift region (n⁻ layer) determines the voltage blocking capability.
The major structural difference between the two devices lies in the backside layer configuration of the drift region.
Figures 1 and 2 show the circuit symbols, device structures, and simplified equivalent circuits of MOSFETs and IGBTs.
In the simplified equivalent circuit of the MOSFET shown in Figure 1, the drift region resistance—which dominates the conduction characteristics of high-voltage MOSFETs—is represented in a simplified form.
In contrast, the simplified equivalent circuit of the IGBT in Figure 2 represents the minority carrier injection mechanism using a diode that corresponds to the pn junction between the p+ collector and the drift region, rather than depicting a full PNP transistor structure.

For more details on MOSFET structures, please refer to the following FAQ:
> What types of structures are there in MOSFETs?

Symbol, vertical structure, and simplified equivalent circuit of a MOSFET, showing the drift region resistance that dominates conduction characteristics in high-voltage MOSFETs.
Figure 1 MOSFET symbol, structure, and equivalent circuit
Symbol, vertical structure, and simplified equivalent circuit of an IGBT, showing a MOSFET structure with drift region resistance and a PN junction representing minority carrier injection.
Figure 2 IGBT symbol, structure, and equivalent circuit

Differences in Operating Principles Between MOSFETs and IGBTs
Due to their structural differences, MOSFETs and IGBTs differ in both the type of charge carriers involved in conduction and their carrier transport behavior.

Operating Principle of MOSFETs
In a MOSFET, applying a gate voltage forms a conductive channel between the source and drain, allowing current to flow.
During conduction, the current is carried only by majority carriers (electrons or holes).
Because MOSFETs operate exclusively with majority carriers, they are classified as unipolar devices, and no carrier storage occurs in the drift region.
As a result, MOSFETs can switch on and off rapidly, enabling high-speed switching operation.

For a detailed explanation of MOSFET operation, refer to the following FAQ:
> Understanding MOSFET Operation Principles and Mechanisms

Operating Principle of IGBTs
In addition to MOS gate channel control, IGBTs utilize minority carrier injection from a p⁺ layer located on the collector side into the drift region.
When a gate voltage is applied, a MOS channel is formed, and minority carriers (holes) are injected into the drift region from the p⁺ collector layer. At the same time, the formation of the MOS channel enables electrons to be supplied from the n⁺ emitter side.
As holes are injected into the drift region, it becomes necessary to maintain quasi-neutrality*, meaning that charge imbalance must be avoided within the semiconductor.
Consequently, the electron density in the drift region increases correspondingly, resulting in a state where both electrons and holes coexist at high densities.
Because both carrier types are present at high concentration, the conductivity of the drift region increases significantly.
This phenomenon is known as conductivity modulation.
Figure 3 schematically illustrates carrier behavior in the drift region during conductivity modulation, based on the structure shown in Figure 2. It shows injected holes from the collector side and electrons supplied from the emitter side coexisting simultaneously within the drift region.
Under normal conditions, carrier concentration in the drift region is determined by the doping concentration and remains in equilibrium. In IGBTs, however, minority carrier injection causes the carrier concentration to greatly exceed this equilibrium level.
This condition, in which injected carriers remain in the drift region for a certain period of time, is referred to as carrier storage.

For further details, see the following FAQ:
> What is conductivity modulation?

Schematic diagram showing carrier flow during conductivity modulation in an IGBT, where holes injected from the p+ collector side and electrons supplied from the n+ emitter side enter the n- drift region.
Figure 3 Carrier movement during IGBT conductivity modulation

* :  Quasi-neutrality refers to a condition within a semiconductor region in which the charges of electrons (−) and holes (+) almost cancel each other, resulting in negligible space charge.

Impact of Operating Principle Differences on Switching Characteristics
Because no carrier storage occurs in MOSFETs, turn-on and turn-off transitions follow changes in gate voltage almost immediately, enabling high-frequency switching.
In contrast, carrier storage occurs in the drift region of IGBTs. During turn-off, the stored minority carriers must be recombined or removed before the current can fully cease. As a result, current continues to flow for a short period after gate turn-off, and the switching speed becomes slower than that of MOSFETs.
This residual current component after turn-off is called tail current.
Tail current is a characteristic phenomenon observed during turn-off of IGBTs, which involve minority carrier storage due to bipolar operation, and it does not occur in unipolar devices such as MOSFETs.

Differences in Conduction Characteristics Between MOSFETs and IGBTs
In high-voltage MOSFETs, the drift region must be thick and lightly doped to achieve sufficient blocking capability. As a result, the resistance component of the drift region becomes the dominant part of the on-resistance, and the conduction voltage drop increases as the current increases.
In contrast, in IGBTs, minority carrier injection causes conductivity modulation, which significantly increases carrier density in the drift region.
Figure 4 presents an example comparing the on-voltage characteristics of IGBTs and MOSFETs.
It shows that MOSFETs have lower on-voltage in the low-current region, whereas IGBTs exhibit lower VCE(sat) in the high-current region.
Thus, the distinct current-dependent behavior of the on-state voltage reflects a fundamental difference in the conduction characteristics of MOSFETs and IGBTs.

Figure 4 Notes
Figure 4 compares the on‑voltage characteristics of an IGBT and a high‑voltage MOSFET
(a Super‑junction MOSFET is shown here as a representative example).
The horizontal axis represents the on-state voltage—VDS(on) for the MOSFET and VCE(sat) for the IGBT—while the vertical axis represents conduction current. Characteristics at ambient temperatures of 25°C and 150°C are shown.
In the low-current region, the MOSFET on-voltage rises from nearly 0V. In contrast, the IGBT on-voltage starts at a higher value due to the voltage component originating from the pn junction, resulting in a forward-voltage offset similar to that of a diode.
In the high-current region, the slope of the IGBT characteristic becomes steeper, and above a certain current level, the IGBT exhibits a lower VCE(sat) than the MOSFET’s VDS(on).

For a detailed explanation of Super-junction MOSFET structures, please refer to the following FAQ:
> What types of structures are there in MOSFETs?

Comparison of on-voltage characteristics between an IGBT and a MOSFET (SJ MOSFET), showing conduction current versus on-state voltage (VDS(on) and VCE(sat)) at ambient temperatures of 25°C and 150°C. The curves indicate lower on-voltage for the MOSFET at low current and lower VCE(sat) for the IGBT at high current.
Figure 4 Comparison of on-voltage characteristics between IGBT and high-voltage MOSFET

Summary: How to Choose Between MOSFETs and IGBTs
Due to differences in structure and operating principles, MOSFETs and IGBTs exhibit different trade-offs in conduction and switching characteristics.
MOSFETs are well suited for low- to medium-voltage applications that require low on-resistance and high-speed switching, particularly in high-frequency operation.
IGBTs, on the other hand, provide superior conduction performance under high-voltage and high-current conditions due to minority carrier injection and conductivity modulation.
However, attention must be paid to switching speed due to tail current caused by carrier storage.
By considering voltage level, current magnitude, and switching frequency requirements, the appropriate device can be selected based on a clear understanding of these trade-offs.

Typical application domains include:

  • MOSFETs
    Low- to medium-voltage applications requiring high-speed switching
    (e.g., switching power supplies, DC-DC converters, high-frequency circuits)
  • IGBTs
    High-voltage and high-current applications with relatively low switching frequency
    (e.g., inverters, motor drives, industrial power conversion systems)

Understanding the trade-offs among conduction characteristics (RDS(on) / VCE(sat)), conduction loss, and switching loss is essential for selecting the most suitable device for each application.

  BJT MOSFET IGBT
Table 1 General comparison of BJT, IGBT, and MOSFET as power elements
Drive type

Current drive

Voltage drive

Voltage drive
Power for driving High Low Low
On state voltage Moderate

Tends to increase in
proportion to
breakdown voltage

Low
Switching speed

Low

High

Moderate
Temperature stability Poor Good Good
Difficulty of achieving high
breakdown voltage		 Moderate High Easy with conductivity
modulation

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