Understanding IGBT Structure:
This post explains about the structure of the Insulated Gate Bipolar Transistor (IGBT). For better understanding of the IGBT structure it is recommended to read the IGBT Basics
- A major limitation of Power MOSFET is the value of RDS(ON) increases as the voltage rating of the device is increased.
- This problem arises because the doping of the drain region of the MOSFET must be reduced as the voltage rating is increased. As the doping is reduced the conductivity of the device is decreased.
- The IGBT tries to overcome this limitation by using a forward biased P-N junction to inject minority carriers into this region so as to increase the conductivity.
- The generic n-channel IGBT structure is shown in the above figure.
- This structure is similar to the structure of MOSFET except the presence of p+ layer( which is connected to the drain terminal).
- The lower p-n junction is forward biased when the device is on and injects holes into the n- layer enhancing its conductivity.
- The doping levels of each IGBT layers are similar to the doping levels of each layer of MOSFET except the body region.
As mentioned in the figure, the IGBT structure has a parasitic thyristor. Turn off of this SCR is undesirable.
- Similar to MOSFET structure, the IGBT does retain the extension of the source metallization over the body region.
- The body source short in the IGBT helps to minimize the possible turn-on of the parasitic thyristor.
- The n+ buffer layer between the p+ drain contact and the n- dirft layer is not important for the operation of the IGBT.
- Some IGBTs are made without this layer which is called as Non-Punch-Through (NPT - IGBT). It is also known as symmetrical IGBT. Because this type of device has equal forward and reverse breakdown voltage.
- IGBTs manufactured with this buffer layer is called as Punch-Through (PT-IGBT). It is also known as asymmetrical IGBT. In this categary, the reverse breakdown voltage is less than forward breakdown voltage.
- The difference between these two types are discussed in a seperate post. Please click here to know more about the difference between NPT-IGBT and PT-IGBT.
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