MOSFETs and bipolar junction transistors (BJTs) are among the most common electronic power switching devices. However, both components have limitations - especially when used in high current applications. IGBTs are an alternative type of device that combines the benefits and features of both the BJT and the MOSFET.
But what is an IGBT and how do they work? In this guide, we will explore insulated-gate bipolar transistors, covering their applications, types and how they differ from other devices.
What is an IGBT?
An insulated-gate bipolar transistor (IGBT) is a type of discrete semiconductor that fundamentally combines the characteristics of both MOSFETs and bipolar transistors. These power modules incorporate the high-speed switching performance of power MOSFETs as well as the high voltage and current handling capabilities of bipolar transistors. In other words, IGBTs can handle large collector-emitter currents with very low gate current drive.
IGBT modules have three terminals and consist of four alternating layers controlled by a metal-oxide-semiconductor gate structure. They are also unidirectional, meaning they can only switch current in the forward direction. This differs from MOSFETs, which are capable of bidirectional current switching.
What are IGBTs used for?
Because these modules combine the benefits of both BJTs and MOSFETs, they are often used in power supply and motor control circuits. They are often found in power electronics applications where demands cannot be met by power MOSFETs and transistors alone.
Similarly, IGBT modules are typically used as electronic switching devices. This is because they combine fast switching with high efficiency, making them ideal for this task. When used as a statically controlled switch, IGBTs have current and voltage ratings similar to those of a bipolar transistor. However, the presence of the isolated gate gives them the advantage as much less drive power is required.
Standard IGBT applications are high voltage and medium speed. They can include
Variable speed control
Switching power supplies (SMPS)
DC-AC inverters
Driving inductive loads
Pulse width modulation (PWM)
AC and DC motor drives
Frequency converters
Traction motor control
How Does an IGBT Work?
As the name suggests, insulated-gate bipolar transistors combine the technology of both MOSFETs and bipolar junction transistors.
The first part of the name refers to the insulated gate like a MOSFET, while the second part refers to the output performance characteristics of a standard bipolar transistor. This suggests that IGBT devices are voltage-controlled like MOSFETs while maintaining the transistors' conduction characteristics and output switching. In summary, IGBTs offer higher voltage operation with more power gain than BJTs and lower input losses than MOSFET. They combine a PNP transistor output with an isolated gate N-channel MOSFET output.
IGBTs are three-terminal transconductance modules. These terminals are emitter, collector and gate. The latter controls the device while the first two are connected to the current and the conductance path.
As with the power BJT, the amount of amplification is determined by the ratio between the input and output signals. However, since the gate is isolated from the current-carrying channel in an IGBT, there is no input current. Therefore, the base current is provided by a MOSFET.
IGBT Construction
IGBT modules consist of four layers of semiconductors glued together. These are
P+ substrate layer (closest to the collector terminal)
N-layer
P layer (closest to the transmitter terminal)
N+ tier (within the P tier, but not available on all devices)
IGBT Types
IGBT transistors can be classified in two ways. This is based on the N+ buffer layer, which is one of the semiconductor layers located inside the P layer, as described above. Devices with this layer are known as punched IGBTs, while those without it are called non-punched IGBTs.
Based on these characteristics, they can also be referred to as symmetrical and non-symmetrical IGBTs. Symmetrical devices have equal forward and reverse breakdown voltage, while unsymmetrical devices have lower reverse breakdown voltage than forward breakdown voltage. For this reason, unsymmetrical IGBTs are often used in DC circuits because there is no requirement to support the voltage in the reverse direction.
Package & Mounting Type
Various package IGBTs are available depending on the specific manufacturer. This will also depend on the type of mounting - for example, through hole, panel mount or surface mount.
FAQ
What are the Advantages of IGBTs?
Because of their hybrid configuration, there are many advantages to using IGBT modules over other types of power transistors. The advantages include:
High voltage capabilities
Simplicity of the driver
Low state power consumption
Low resistance
Fast switching speed
What is the Difference Between IGBT and MOSFET?
While MOSFETs and IGBTs are similar components, both transistors have important differences. A few of the key differences between IGBTs and MOSFETs are summarized below.
IGBT power modules are preferred for high voltage applications as they are specifically manufactured to operate under high voltage. To put this into context, IGBTs have a typical voltage rating of around 1400V, while MOSFETs are much lower rated at around 600V.
Despite their similar structure, the two device types have different terminals and the three terminals of an IGBT (emitter, collector, gate) are different from those of a MOSFET (source, drain, gate).
In this table you can see a comparison between the two types of devices:
Characteristic IGBT Power MOSFET
Current Likes High Low
Voltage measurement Very high High
Switching Speed Medium Fast
Input Impedance High High
Output impedance Low Medium
How Can I Control IGBTs?
IGBT transistors can be turned on or off by activating or deactivating the gate terminal. When in the on state, a positive input voltage must be applied across the gate that signals the transmitter to keep the device on. Since these devices are voltage controlled, only a small voltage is required at the gate to maintain transmission through the IGBT module. In the off state, changing the input gate to receive a zero or negative signal will trigger the device to turn off.
How Do You Test IGBTs?
IGBT testing is important to ensure that the device and diodes are in good condition. Testing can also determine if the gate terminals are properly insulated.
IGBT modules can be easily tested with a multimeter, but if more extensive circuit diagnostics are required, it is always recommended to consult an expert. Also note that these modules can be highly sensitive to ESD and therefore anti-static protective equipment should always be used when handling IGBT transistors.