The IGBT has combine
characteristics of the MOSFET and BJT
MOSFET
The switching speed of
the MOSFET is high
The on-state voltage drop per unit area is very high therefore it has high on state power loss.
BJT
Low switching speed.
Low on state power loss
How many terminals the IGBT consists of?
Three terminals : Collector,
Emitter and Gate
What is structure difference between n channel IGBT and n
channel MOSFET?
The IGBT consists of P+
layer at the drain side that does not exist in the MOSFET.
Whether the IGBT is voltage controlled or current controlled
device?
Voltage controlled
Why the gate
– source structure of the IGBT is kept highly inter – digitated?
To reduce emitter /
source current crowding
State the other name of IGBT.
Insulated gate transistor
( IGT ) OR Bipolar MOS transistor OR Conductivity modulated field
effect transistor ( COMFET )
Why the vertical oriented structure of the IGBT is used?
Vertical structure
The vertical oriented
structure of the IGBT is used in order to maximum area available for current
flow. It will also reduce resistance offered to the current flow therefore
total power loss is reduced.
Why the on-state losses in the IGBT are less than that of
MOSFET?
ON state power loss
The IGBT consist of conductivity modulation which does not exist in the MOSFET. The on-state losses due to effect of conductivity modulation in the IGBT are less than that of MOSFET.
Why the P+ region in the n channel IGBT is called as
injecting layer?
Injecting layer
The structure of the n
channel IGBT consists of P+ region close to n- drift
region. This P+ region makes PN junction diode with drain layer and
injects minority charge carriers (holes) in it. Therefore, it is called as
injecting layer.
Which resistance the equivalent circuit of the IGBT consists of?
The IGBT consists of
drift resistance and body spreading resistance.
What do you mean by the symmetric IGBT and asymmetric IGBT?
Asymmetric IGBT
It consists of n+ buffer layer.
Symmetric IGBT
It does not consist of n+ buffer layer.
Which parameter greatly affects the forward blocking voltage of
the n channel IGBT?
Doping level and width of n- layer
What do you mean by Punch through IGBT and non-Punch through
IGBT?
Punch through ( PT ) IGBT
The structure of the n
channel IGBT consists of heavily doped n+ region between P+
injecting layer and n- drift layer. It is called as punch through
IGBT.
Non punch through ( NPT )
IGBT
The structure of the n
channel IGBT does not consist of heavily doped n+ region between P+
injecting layer and n- drift layer. It is called as punch through
IGBT.
Describe the main difference between punch through IGBT and Non
punch through IGBT.
Punch through IGBT
Low on state voltage drop
Low reverse breakdown
voltage
Non punch through IGBT
High on state voltage
drop
High reverse breakdown voltage
Which is controlling parameter in the IGBT?
Gate – emitter voltage
Why the on-state voltage drops across IGBT is lower than that of
the MOSFET?
Due to conductivity
modulation
Explain
the conductivity modulation in the IGBT.
OR
Why the on-state voltage in the IGBT is lower than that of
MOSFET?
Conductivity modulation
When the forward voltage
is applied between collector and emitter, junction J3 between n+
layer and p+ layer becomes forward biased and it will inject holes
in the n+ buffer layer. The electrons are injected into n-
drift layer from n+ layer due to creation of inversion layer. The
electrons injected in the n- layer create space charge which will
attract holes that are injected by p+ layer. The double injection
takes place in the n- drift layer from both sides. The conductivity
of n- drift layer increases due to double injection therefore its
resistance decreases. The on-state
voltage drop decreases due to conductivity modulation in the IGBT. There is no
conductivity modulation in the MOSFET therefore the on-state voltage in the
MOSFET is higher than that of the IGBT.
Why the
resistance of the IGBT remains constant under different temperature conditions?
OR
Why secondary breakdown does not take place in the IGBT?
Resistance temperature
co – efficient
The resistance
temperature co – efficient of the IGBT is flat therefore it has no effect of
temperature variation. The resistance of the IGBT remains constant and
secondary breakdown does not take place due to above reason.
What do you mean by the body spreading resistance?
Body spreading resistance
The base of the NPN
transistor is shorted with emitter-by-emitter metallization. However due to
imperfect short, there is always resistance between them. This resistance is
called as Body spreading resistance.
Describe the effect of body spreading resistance on the n
channel IGBT.
Effect of body spreading
resistance
The equivalent circuit of the n channel IGBT consists of body spreading resistance between base and emitter of the NPN transistor. If the output current is high, the voltage drops across body spreading resistance forward biased the NPN transistor. This will result in latching up of PNPN structure. Once the structure of the IGBT latch up, the gate losses its control and device may destroy due to high power loss.
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