IGBT Interview Question Answer - 1

1	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
2	How many terminals the IGBT consists of?
	Three terminals : Collector, Emitter and Gate
3	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.
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.
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.
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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