V-I Characteristics

 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 V_CE below the  breakdown level  V_B. When V_C < V_B  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 V_I  characteristics of IGBT  is shown  in figure  it show the relation  between  collector current  t_c and collector to emitter voltage V_CE  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 V_I  characteristics. The junction J₂ blocks the forward  voltage when the IGBT is off.