In the normal mode of operation of an IGBT a positive voltage is applied to the collector relative to emitter. When the gate is at zero potential with respect to E no collector current IC flows for collector voltage VCE below the breakdown level VB. When VC < VB and the gate voltage exceeds the threshold value VGT, electrons pass into the N- region ( base of the P-N-p transistor).
Figure V- I characteristics of IGBT
These electrons lower the potential of the N region biasing the P+ - N_ junction thereby causing holes to be injected from the P+ substrate into the N_ epi-layer region. The excess electrons and holes modulate the conductivity N+ region which dramatically reduces the on resistance of the device. With zero gate bins the forward characteristic of IGBT shows very low current (<1bA) where it breaks up sharply to much larger current levels with only a slight increase in voltage. The VI characteristics of IGBT is shown in figure it show the relation between collector current tc and collector to emitter voltage VCE for different values of gate to emitter voltage VGE.
The junction J1 blocks reverse voltage. An IGBT without N+ buffer layer has higher reverse blocking capability. Therefore an IGBT required for blocking high reverse voltage does not have N+ buffer layer. The reverse blocking voltage is shown as the VRM on the VI characteristics. The junction J2 blocks the forward voltage when the IGBT is off.