The Insulated Gate Bipolar Transistor (IGBT) is one of the most important power semiconductor devices in modern electronics. Combining the advantages of MOSFETs and BJTs, IGBTs have become indispensable in high-power applications such as electric vehicles, renewable energy systems, smart grids, and rail transit. This article explains the definition, structure, working principle, advantages, applications, and key considerations for selecting an IGBT.
Definition
An IGBT (Insulated Gate Bipolar Transistor) is a composite, fully-controlled, voltage-driven power semiconductor device that integrates the high input impedance of MOSFETs with the low conduction voltage drop of BJTs. This combination gives IGBTs high voltage and current handling capabilities along with efficient switching.
Structure
An IGBT typically consists of four semiconductor layers: a P-type emitter, an N-type base, an N-type drift region, and a P-type collector. The MOSFET structure (P-emitter/N-base) controls current flow, while the BJT structure (N-drift/P-collector) provides high current conduction. The insulated gate at the top controls the MOS channel, which in turn enables or disables the BJT conduction.
HIITIO IGBT Power Modules
Working Principle
An IGBT operates by activating or deactivating its gate terminal to turn it on or off. If a positive input voltage is applied to the gate, the emitter keeps the driver circuit on. On the other hand, if the voltage at the IGBT’s gate terminal is zero or slightly negative, the circuit is turned off.
Because an IGBT functions as both a BJT and a MOSFET, the amount of amplification it achieves is the ratio between its output signal and the control input signal.
For a traditional BJT, the amount of gain is roughly the same as the ratio of the output current to the input current, which we call beta.
For a MOSFET, on the other hand, there is no input current because the gate terminal is isolated from the main current-carrying channel. The gain of an IGBT is determined by dividing the change in output current by the change in input voltage.
IGBT Structure Diagram
As shown in the figure, an N-channel IGBT turns on when the collector is at a positive potential relative to the emitter, while the gate is also at a sufficiently positive potential relative to the emitter (>V GET ). This condition causes an inversion layer to form directly beneath the gate, thereby forming a channel and allowing current to flow from the collector to the emitter.
The collector current Ic in an IGBT consists of two components, le and lh. le is the current flowing from the collector to the emitter due to injected electrons passing through the injection layer, drift layer, and the resulting channel. Ih is the hole current flowing from the collector to the emitter through Q1 and the bulk resistance Rb. Therefore, even though lh is almost negligible, Ic ≈ le.
A unique phenomenon observed in IGBTs is known as IGBT latch-up. This occurs when the collector current exceeds a certain threshold (ICE ). In this condition, the parasitic thyristor becomes latched, and the gate terminal loses control of the collector current. Even if the gate potential is reduced below V GET, the IGBT cannot turn off. Now to turn off the IGBT, we need a typical commutation circuit, such as the case of forced commutation of thyristors. If the device is not turned off quickly, it may be damaged.
The figure below explains the working principle of IGBT very well and describes the entire device operating range of IGBT.
IGBT Working Principle Diagram
An IGBT operates only when there is a voltage applied to the gate terminal—this is the gate voltage, or VG. As shown in the figure above, once the gate voltage (VG) is present, the gate current (IG) increases, which in turn increases the gate-to-emitter voltage (VGE).
Thus, the gate-to-emitter voltage increases the collector current (IC). Consequently, the collector current (IC) reduces the collector-to-emitter voltage (VCE).
Note: IGBTs have a diode-like voltage drop, typically on the order of 2V, which increases only with the logarithm of the current.
IGBTs use a freewheeling diode to conduct reverse current. The freewheeling diode is placed between the collector and emitter terminals of the IGBT.
Schematic diagram of the applicable scope of various power devices based on output capacity and operating frequency：
Types of IGBTs
- Punch-through (PT) IGBTs: With an N+ buffer layer, they are suitable for high-speed switching.
- Non-punch-through (NPT) IGBTs: Without the buffer layer, offering higher robustness and symmetrical blocking capability.
- Symmetrical IGBTs: Same forward and reverse breakdown voltage, suitable for AC circuits.
- Asymmetrical IGBTs: Lower reverse breakdown voltage, mainly used in DC applications.
How to Choose the Right IGBT
When selecting an IGBT for your application, consider the following factors:
- Voltage Rating (Vces): Choose at least 20–30% higher than the maximum circuit voltage.
- Current Rating (Ic): Ensure sufficient margin above the expected load current.
- Switching Frequency:
- For high-frequency applications (inverters, SMPS), prioritize low switching losses.
- For low-frequency, high-power applications (traction, grid), prioritize low conduction losses.
- Thermal Management: Consider junction temperature, heat dissipation, and package design.
- Application Type:
- EVs & charging: fast switching, high efficiency.
- Smart grid: high-voltage endurance, reliability.
- Rail transit: high robustness and thermal stability.
- Cost vs. Performance: Higher-end modules offer lower losses but at greater cost. Balance efficiency and budget.
Rule of thumb: For industrial applications, always consult the datasheet, simulate under load conditions, and consider long-term reliability in your system design.
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Advantages and Disadvantages Analysis
Advantages
- High voltage and current handling capacity
- High input impedance
- Simple and cost-effective gate driving
- Low conduction resistance
- High current density with a smaller chip size
- High power gain compared to BJTs and MOSFETs
- Fast switching speed
- Reliable and safe operation
Limitations
- Slower switching speed than MOSFETs
- Unidirectional, cannot handle AC without extra circuitry
- Limited reverse blocking capability
- More expensive than MOSFETs and BJTs
- Possible latch-up issues due to PNPN structure
Explore more insights in our related post:
Advancing Power Electronics: The Next Generation of Press-Pack IGBTs
Applications of IGBTs
1. New Energy Vehicles
- Motor Drive (DC/AC Inverter): Powers EV motors.
- On-board AC Control: Manages vehicle HVAC systems.
- Charging Stations: Used as switching devices in fast chargers.
2. Smart Grid
- Generation: Wind and solar inverters.
- Transmission: HVDC and FACTS applications.
- Distribution: Core component in power electronic transformers.
- End Use: Appliances like microwaves, LED drivers, and HVAC systems.
3. Rail Transit
IGBT-based traction inverters are at the heart of AC drive systems, making them indispensable in trains, metros, and auxiliary power systems.
FAQs
Q1: What is the difference between IGBT and MOSFET?
IGBTs are better for high-voltage applications (up to 1400V), while MOSFETs are faster but limited to lower voltages (around 600V).
Q2: How do you control an IGBT?
By applying a positive voltage to the gate (turn on) and removing or applying a negative voltage to the gate (turn off).
Q3: How do you test an IGBT?
Using a multimeter to check insulation, gate function, and diode integrity. Handle with ESD protection.
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