1
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How IGBT is superior over the BJT and MOSFET?
|
|
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
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2
|
How many terminals the IGBT consists of?
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Three
terminals : Collector, Emitter and Gate
|
3
|
What is structure difference between n channel IGBT and n
channel MOSFET?
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The IGBT
consists of P+ layer at the drain side that does not exist in the
MOSFET.
|
4
|
Whether the IGBT is voltage controlled or current controlled
device?
|
|
Voltage
controlled
|
5
|
Why the gate – source structure of the IGBT is kept highly
inter – digitated?
|
|
To reduce emitter
/ source current crowding
|
6
|
State the other name of IGBT.
|
|
Insulated gate
transistor ( IGT )
OR
Bipolar MOS transistor
OR
Conductivity
modulated field effect transistor ( COMFET )
|
7
|
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.
|
8
|
Why the on state losses in the IGBT are less than that of
MOSFET?
|
|
ON state power loss
- The IGBT
consists 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.
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9
|
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.
|
10
|
Which resistance the equivalent circuit of the IGBT consists
of?
|
|
The IGBT
consists of drift resistance and body spreading resistance.
|
11
|
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.
|
12
|
Which parameter greatly affects the forward blocking voltage
of the n channel IGBT?
|
|
Doping level
and width of n- layer
|
13
|
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.
|
14
|
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
|
15
|
Which is controlling parameter in the IGBT?
|
|
Gate – emitter
voltage
|
16
|
Why the on state voltage drops across IGBT is lower than that
of the MOSFET?
|
|
Due to
conductivity modulation
|
17
|
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.
|
18
|
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.
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19
|
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.
|
20
|
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 drop 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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