What Is IGBT?
IGBT is an insulated gate bipolar transistor. The name is shortened to IGBT.
An equivalent circuit can be considered a composite transistor circuit configuration with an N-channel MOSFET at the input and a PNP-type bipolar transistor at the output. On the other hand, since the structure can be said to have a MOSFET in the base of the bipolar transistor section, it can generate a very large output current for a small current.
It is a high-performance semiconductor with a higher breakdown voltage and lower loss than the MOSFET that serves as the base. IGBT was developed in Japan in the 1980s, and its structure at that time was called a punch-through type.
In recent years, as wafer processes have progressed, IGBT devices have become smaller and less expensive, and non-punch-through and field-stop IGBT devices are now being manufactured.
Uses of IGBTs
IGBTs are commonly used in variable-speed drives and power converters due to their high speed under high-power operating conditions.
They are also used extensively in inverter circuits for induction cooktops, washing machines, and air conditioners, as well as in the power control of large home appliances such as printers. With the recent trend towards energy conservation, the use of IGBTs, which can reduce power loss, is further expanding.
Principle of IGBTs
IGBTs have the structure of a MOSFET at the input and a bipolar transistor at the output, as explained at the beginning of this article, and have characteristics that are a combination of the features of each.
Because of the two types of carriers, its switching speed is slower than that of a MOSFET but faster than that of a bipolar transistor, and its withstand voltage is improved over that of a MOSFET. When voltage is applied from the gate, which is the input part of the terminal, current flows from the MOSFET and conducts to the P-type semiconductor, which in turn amplifies a small amount of current, as is the nature of bipolar transistors, allowing a large current to flow between the emitter and collector.
In addition, conductivity modulation occurs as in a bipolar transistor so that on-resistance can be lowered and current density can be increased. Since a constant voltage drop occurs between the collector and emitter, losses can be lower than with MOSFETs when the current is high.
Other Information on IGBTs
1. About Inverter Circuits Applying IGBTs
An inverter circuit is a DC-to-AC conversion circuit used in pairs with an AC-to-DC converter circuit. IGBTs are used in this inverter circuit to output AC with a different voltage and frequency.
IGBTs are switched to adjust the on/off interval and pulse width. By generating and shaping different pulse waves, the pulse wave is made closer to a sine wave. This is called pulse width modulation, and IGBTs are often used here.
The functions of home appliances are controlled by changing the speed of the motor through frequency conversion by pulse width modulation. IGBTs are widely used in home appliances such as air conditioners, refrigerators, industrial motors, and computer power supplies.
2. Difference between IGBTs and MOSFETs
IGBTs are often described as a combination of MOSFETs and BJTs (bipolar junction transistors), but they have some drawbacks when compared to MOSFETs. IGBTs have a rising voltage that has an offset due to their configuration, especially in the low-current range. In general, MOSFET devices have lower Vds than IGBTs, especially in the low current range.
IGBTs are mainly focused on the medium to the high current range, so they exhibit lower on-resistance than MOSFETs in this range. Still, for applications where efficiency in the low power range is essential, MOSFETs have better characteristics. = 2V, MOSFETs are superior in efficiency, while IGBTs are superior at higher voltages.
3. About IGBT Modules
IGBTs are complex devices, and it takes a lot of effort to assemble them so that their operation can be controlled from scratch. Therefore, IGBT modules that combine signal processing, amplification, protection circuits, parasitic diodes, and other components of the control part into a composite module are widely commercialized.
Since IGBTs are transistors that are prone to breakdown if their SOA (Safety Operation Area) or absolute maximum ratings are exceeded, some of them have built-in protection circuits. IGBTs were developed to achieve both withstand voltage and switching speed and have been improved over the years. In this power device area, power semiconductor devices using new compound semiconductor materials such as SiC and GaN have recently begun to be introduced.
These next-generation power semiconductor devices enable faster-switching operations than IGBTs and have superior breakdown voltages, so research and development of these devices have been increasingly active in recent years. However, there are still issues to be cleared, such as cost and supply, and they are not expected to replace the entire current IGBTs market area.