An insulated-gate bipolar transistor (IGBT) is a three-terminal semiconductor device it is a hybrid of MOSFET and BJT for high efficiency and fast switching. The power BJT has the advantage of low on state power dissipation but it cannot be switched at faster rates due to longer turn-off time, whereas MOSFETs have a very high switching speed but their power handling capacity is not as good as that of BJTs.
The power MOSFET can be switched at much higher frequency but has a drawback of higher on state power loss.
Therefore we made an insulated gate bipolar transistor (IGBT) to combine BJTs and MOSFETS monolithically on the same silicon wafer to develop a new device that will have the best qualities of both, BJT and MOSFET.
Other names for this device are GEMFET, COMFET (conductivity modulated field-effect transistor), IGT (insulated gate transistor), and bipolar mode MOSFET or bipolar MOS transistor.
Features of IGBT:
The important features of IGBT are as follows:
Low on-state voltage drop.
Low on-state power loss
It has a higher switching frequency than that of a power BJT.
IGT has the best qualities of BJT and MOSFET.
Basic Structure of IGBT:
The vertically oriented structure of IGBT is as shown in Figure.
Read this article to understand the VI characteristics of IGBT.
Like all other devices, IGT also uses the vertically oriented structure to maximize the area available for the current flow.
This will reduce the resistance offered to the current flow and hence the on-state power loss taking place in the device.
The IGT also uses a highly interdigitated gate-source structure to reduce the possibility of source/emitter current crowding.
The doping levels used in different layers of IGBT are similar to those used in the comparable layers of the power MOSFET structures except for the body region. The main difference in the structure of IGBT as compared to that of a MOSFET is the existence of a p+ (injection layer or drain layer) layer that form’s the drain of the IGBT.
This device also uses the n-type drain drift layer which improves its breakdown voltage capacity. This is the same as that in case of power MOSFETs.
It is also possible to make a p-channel IGBT by changing the doping type in each of the layers of the device.
The n+ buffer layer is not a must (it is an optional layer) for the operation and some IGBTs do not have it. IGBTs without a buffer layer are known as symmetric IGBTs whereas those with the buffer layer are called asymmetric IGBTs.
The n+ buffer layer improves the operation of IGBT in two important aspects:
It reduces the on-state voltage drop across the device
It shortens the turn-off time.
But the drawback is that the buffer layer reduces the reverse blocking capacity of the device to a great extent.
The circuit symbol:
The circuit symbols for an n channel IGBT are as shown in Figure. It is a three-terminal device, gate being the control terminals.
The directions of the arrowheads will reverse in a p channel IGBT.
The symbol shown in Figure is almost identical to that used for the n channel power MOSFET, but with the addition of an arrowhead in the drain terminal pointing into the device.
The symbol shows that the insulated-gate bipolar transistor is considered to be a BJT with MOSFET gate input.
This device has a collector and emitter instead of a drain and source. The controlling terminal however is the gate terminal(G).
This symbol indicates that it has output characteristics similar to power BJT and input characteristics similar to the power MOSFET.
Ratings of IGBT:
Here are the ratings of IGT
Applications of IGBT:
Some of the important applications of insulated-gate bipolar transistor are:
Switching mode power supplies (SMPS)
UPS systems (inverters)
AC motor controllers
Advantages of IGBT over BJT:
It is a voltage-controlled device so that gate driving is easy.
It can switch at a higher frequency than BJT. Typically =20kHz.
They can be easily connected in parallel. The second breakdown does not take place.
No need snubber circuits for their protection.
Advantages of IGBT over MOSFET:
On state voltage drop across IGBT is less than that across MOSFET.
Low on-state power dissipation.
IGBTs can handle larger power than MOSFETs.