Insulated Gate Bipolar Transistor ( IGBT ) Interview Question Answer - 2

1	How to reduce body spreading resistance in the IGBT?
	The body spreading resistance in the IGBT is reduced by making body region heavily doped.
2	How the latching problems in the IGBT can be avoided?
	The latching problem in the IGBT is avoided by modifying doping level and geometry of the body layer.
3	When the inversion layer formed in the IGBT?
	When the gate – emitter voltage exceeds threshold voltage, the inversion layer is formed in the p type body region ( for n channel IGBT)
4	Why the doping level of the drift region is kept low in the n – channel IGBT?
	Drift region The doping level and thickness of the drift layer determines the current amplification factor of the PNP transistor.  The doping level and thickness of the drift layer is kept low resulting most of the current flows through the MOSFET. Therefore the voltage drop across body spreading resistance reduces and possibility of static latch up of the IGBT is eliminated.
5	When the IGBT is operated under cut off region?                      OR Explain the term : Cut off region
	Cut off region When the gate – emitter voltage is less than the threshold voltage, the collector – emitter voltage almost equal to supply voltage and only leakage current flows through the device. This is called as cut off region of the IGBT.
6	Why the maximum reverse voltage in the non – punch through IGBT is very small?
	Due to heavily doped n+ drift layer
7	Explain the term : Active region
	Active region When the gate – emitter voltage is greater than the threshold voltage, the IGBT operated in the active region.  The collector current in the region depends upon transfer characteristic of the IGBT.
8	In which operating region the voltage across IGBT remains constant?
	Saturation region
9	Explain the term : Forward breakdown voltage
	Forward breakdown voltage It is collector to emitter voltage at which avalanche breakdown occurs in the IGBT.
10	Why the IGBT is operated below forward breakdown voltage?
	Importance of the forward breakdown voltage The voltage across the IGBT and current passes through IGBT is high at forward breakdown voltage.  This will result in high power loss in the device and it may cause damage it.  Therefore the IGBT is always operated below forward breakdown voltage.
11	How the dynamic latch up of the parasitic thyristor in the IGBT avoided?
	The gate emitter voltage of the IGBT is kept negative during its turn off period in order to avoid dynamic latch up of the thyristor.
12	How the punch through IGBT and non punch through IGBT affect the tail current?
	Tail current The punch through IGBT shortens the duration of the tail current time.  The n+ buffer layer acts as sink for holes and it removes the holes from the drain drift layer therefore the tail time is reduced.  The tail current in the non punch through IGBT is reduced by minimizes the magnitude of the current during failing interval. This is done by designing the MOSFET of the IGBT such that it carries as much as of the total current.
13	Why IGBTs can be easily connected in parallel without any compensating device?
	The IGBTs can be easily connected in parallel without any compensating devices due to flat resistance temperature coefficient.
14	Describe the advantages of flat temperature coefficient of resistance in the IGBT.
	Due to flat temperature coefficient of resistance, the resistance of the IGBT remains constant under all temperature conditions Secondary breakdown does not take place The parallel operation of the IGBTs can be possible without any compensating devices.
15	State the advantages of the IGBT over the BJT.
	Advantages of  the IGBT over the BJT The IGBT is voltage controlled device so gate drive circuit is very simple. The IGBT can operate at higher frequency. Secondary breakdown does not take place Snubber circuit does not require
16	Describe the different modes of latching in the IGBT.
	Latching modes in the IGBT Static latch up Dynamic latch up
17	Give reason : The switching losses in the IGBT is determined only by turn off losses for pure inductive load.
	Switching losses The turn on losses in the IGBT is very small because the transistor is turned on at zero current therefore the total switching losses consists of only turn off losses particularly at pure inductive load.

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