Operation Principle of IGBT

Principle of Operation of IGBT

The principle of operation of IGBT is similar to that of a MOSFET. The operation can be divided into two parts :

1. Creation of inversion layer in IGBT

The operation of IGBT is based on the principle of creation of inversion layer which is same as that for the power MOSFET.
In IGBTS also when the positive gate to source voltage V_GS is greater than V_GS (threshold), n type inversion layer is created beneath the SiO₂ (oxide) layer as shown in Figure (a).
Due to the formation of n type induction layer in the p type body layer, a channel is formed ( n⁺n n^-) which helps to establish the electron current as shown in Figure (a).
Figure (a)

The only difference between the MOSFET and IGBT is that there is no "conductivity modulation" of drift layer in MOSFET.
Therefore the on state resistance R_{DS (on)} and hence the on state power loss is very high in MOSFET. In IGBT however the conductivity modulation takes place which reduces the on state loss as explained as follows:

2. Conductivity modulation of IGBT

In IGBT the conductivity modulation of the n^- drift layer takes place.
The effect of conductivity modulation is reduction in the on state resistance and hence the on state power loss. Therefore the on state losses in IGBT are less than that in MOSFET.
The conductivity modulation in the n^- drift layer can be explained with the help of Figure (a) and (b).
Due to the application of forward voltage between drain (collector) and source (emitter) the junction J₃ is forward biased. Due to the creation of inversion layer, electrons from the source are injected into the n^- drift layer via the n⁺ p n^- channel. (Refer to Figure (b)).
As the junction J₃ is already forward biased, it will inject holes into the n⁺ buffer layer from p⁺ layer. The electrons injected in the n^- drift layer create a space charge which will attract holes from the n⁺ buffer layer which were injected by the p⁺ layer.
In this way "double injection" (of electrons and holes) takes place into the n^- drift region from both sides as shown in Figure (b).
Figure (b)
This increases the conductivity of the drift region and reduces the resistance to its minimum. In this way the conductivity modulation will reduce the on state voltage across the IGBT.