Page 76 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 8 Fig 9

Operation of IGBT
When V > V (threshold), then the channel of electrons is formed beneath the gate as in Fig 10. These electrons
GS
GS
attract holes from p+ layer. Hence, holes are injected from p+ layer into n- drift region. Thus hole / electron current
starts flowing from collector to emitter. When holes enter p-type body region, they attract more electrons from n+
layer. This action is exactly similar to MOSFET.
Fig 11 shows the structure of IGBT showing how internal MOSFETs and transistors are formed. The MOSFET is
formed with input gate, emitter as source and n- drift region as drain. The two transistors T and T are formed
2
1
as in Fig 11. The holes injected by the P+ injecting layer go to the n- drift region. This n- drift region is base of T
1
and collector of T . The holes in the n- drift region further go to the p - type body region, which is connected to the
2
emitter. The electrons from n+ region (which is emitter) pass through the transistor T2 and further in the n- drift
region. Thus holes and electrons are injected in large amounts in n- drift region. This reduces the resistance of
the n- drift region. This is called conductivity modulation of n- drift region. Note that such conductivity modulation
does not exist in MOSFET. The connection of T and T is such that large amount of hole/electrons are injected
2
1
in n- drift region.
Fig 10 Fig 11

The action of T and T is like SCR which is regenerative. The gate serves as trigger for T through internally formed
1
2
1
MOSFET. Fig 12 shows the equivalent circuit. In this figure observe that when gate is applied (V > V (th)), the
GS
GS
internal equivalent MOSFET turns on. This gives base drive to T . Hence T starts conducting. The collector of T
1
1
1
is base of T . Therefore T also turns on. The collector of T is base of T . Thus the regenerative loop begins and
2
2
2
1
large number of carriers are injected in n- drift region. This reduces the on- state loss of the IGBT just like BJT. This
happens due to conductivity modulation of n- drift region.
63
CITS : Power - Electrician & Wireman - Lesson 60-69