Power Semiconductor Devices Interview Questions and Answers:

1. Give the broad classification of power semiconductor devices.

Ans. Power semiconductor devices can be broadly classified into three categories viz. power diodes, thyristors and transistors.

2. Which are the members of “thyristor family”?

Ans. The member of thyristor family are SCR, diac, triac, Shockley diode, SCS, SBS, SUS, CSCR, LASCR, LAS, LASCS, GCS, GTO, etc.

3. How is current limited in conducting state of an SCR?

Ans. The current in conducting state of an SCR is controlled by ex­ternal impedance.

4. What is the effect of negative gate current on a normal SCR?

Ans. Negative gate current will increase the holding current of the SCR.

5. What is false triggering ?

Ans. False triggering is unintended turn-on of an SCR either through gate due to noise pick-up or excessive anode voltage.

6. What are the factors on which `turn-off time’ of an SCR de­pends?

Ans. Turn-off time of an SCR depends upon doping densities, re-combination time, junction temperature, on-state current, rate of decay of on-state current.

7. How the forced turn-off of an SCR is different from natural turn-off?

Ans. In forced turn-off of an SCR current is brought below the hold­ing current by an additional circuit employing energy storage elements while in natural turn-off, it is the natural voltage/cur­rent variation.

8. Why ‘pulse triggering’ is preferred and when does it fail?

Ans. Pulse triggering is preferred as it is effective to turn on an SCR keeping the device gate dissipation low and ensures a fast turn-on keeping the di/dt stress on the device low.

Pulse triggering shall fail if the pulse is of short duration and the load circuit contains large inductance. With highly induc­tive load, the device current builds up slowly. If the pulse dura­tion is small and the anode current is not able to build up to latching current, I_L, the device turns off on removal of gate pulse.

9. Why is it necessary to keep supply voltage much less than breakover voltage Of an SCR ?

Ans. The supply voltage is kept much less than breakover voltage of an SCR otherwise the device will get damaged.

10. What is meant by breakover voltage of an SCR?

Ans. The minimum forward voltage, with gate open, at which SCR starts conducting heavily (i.e., turns on) is called the breakover voltage.

11. What is maximum on-state voltage in reference to an SCR?

Ans. Maximum on-state voltage is the maximum value of the voltage appearing across the SCR during conduction.

12. What is meant by forward current rating of an SCR?

Ans. The maximum value of anode current, that an SCR can handle safely (without any damage) is called the forward current rat­ing of the SCR.

13. What is holding current in an SCR?

Ans. Holding current is the minimum on-state current required to keep the SCR in conducting state without any gate drive.

14. Define latch current of an SCR.

Ans. Latching current is the minimum device current, which must be attained by the device, before the gate drive is removed.

15. Explain the requirements of an SCR to be triggered by a gate pulse.

Ans. Requirements for an SCR to be triggered by a gate pulse are as follows:

1. SCR should be in forward bias mode.
2. Gate should be positive w.r.t. cathode.
3. The gate pulse width should be chosen to ensure anode current more than latching current.

16. What do you understand by string efficiency related to thyristors?

Ans. The string efficiency is a term that is used for measuring the de­gree of utilization of SCRs in a string. String efficiency of SCRs connected in series/parallel is given as

String efficiency = Total voltage/current rating of the whole string Individual voltage/current rating of SCR x Number of SCRs connected in series/parallel in the string

17. Give the typical values of current ratings of SCR.

Ans. Forward breakover current—less than a few hundred micro­amperes.

Peak forward current 30 A to over 100 A.

Holding current—a few mA to few hundred milliamperes.

18. What are bilateral devices ?

Ans. Bilateral devices are those devices which switch symmetrically with forward and reverse bias. Triac and diac fall under this category.

19. What is triac?

Ans. The triac is a three terminal, four layer bidirectional semicon­ductor device. It incorporates two SCRs connected in inverse parallel with a common gate terminal in a single chip. Triac is an abbreviation for a triode ac switch.

20. Why force commutation cannot be applied to triac?

Ans. Because the triac is a bidirectional device i.e., it can conduct in either direction, forced commutation by reverse biasing cannot be applied.

21. What is diac ?

Ans. A diac is a P-N-P-N structured four-layer, two-terminal semi­conductor device. This is just like a triac but without gate ter­minal. Diac is an abbreviation for a diode ac switch.

22. How Shockley diode is switched on ?

Ans. Shockley diode has no trigger inputs, the only way to switch the device on is to increase the anode to cathode voltage to the switching voltage equivalent to SCR forward breakover volt­age.

23. What is silicon controlled switch?

Ans. Silicon controlled switch (SCS), like an SCR, is a unilateral, four layer, three junction P-N-P-N silicon device with four electrodes namely cathode, cathode gate, anode gate and an­ode.

24. What is GTO?

Ans. Gate turn-off switch (GTO), like an SCR, is a four layer, three junction semiconductor device with three external terminals (anode, cathode and gate).

25. How GTO can be turned on and turned off?

Ans. GTO can be turned on and turned off by applying positive pulse and negative pulse respectively to the gate terminal.

26. What is SITH ?

Ans. A static induction thyristor (SITH) is a self controlled GTO ­like on-off device.

27. What are the advantages of MCT ?

Ans. MCT has the advantages of (i) low switching losses (ii) high gate input impedance (iii) low forward voltage drop during con­duction (iv) low reverse voltage blocking capability and (v) fast turn-on and turn-off times. However, it is incapable of with­standing a high reverse voltage.

28. Give the classification of power transistors.

Ans. The power transistors can be classified broadly in four catego­ries: BiTs, MOSFETs, SITs, IGBTs.

29. What is second breakdown effect in power BJTs ?

Ans. Slight non-uniformities in current density develop local regions of increased heating that reduces the resistance of the semiconductor material, which in turn enhances the current in those regions. This effect results in positive feedback, and the current continues to rise, causing a further increase in temperature, until the semiconductor material may actually melt, creating a short-circuit between the collector and emitter and causing a permanent failure. This phenomenon is known as second breakdown and occurs in BJT operating at high voltage and large current.

30. Why Darlington pair configuration is used in BJTs ?

Ans. For achieving high current gain.

31. What is SIT ?

Ans. SIT stands for static induction transistor and it is a solid state version of vacuum triode valve which is used for high frequency and high power applications.

32. What is IGBT ?

Ans. IGBT (insulated gate bipolar junction transistor) is a new high conductance MOS controlled power switch and incorporates some of the best features of power MOSFETs and power tran­sistors.

33. Give the applications of IGBTs.

Ans. IGBTs are finding increasing applications in medium power applications such as dc and ac motor drives, power supplies, solid-state relays, and contractors.

34. What is unijunction transistor ?

Ans. Unijunction transistor (UJT) is a two-layer, three-terminal solid-state (silicon) switching device.

35. What does UJT stand for? Justify the name UJT.
Ans. UJT stands for unijunction transistor. It essentially consists of a lightly doped N-type silicon bar with a small piece of heavily doped P-type material alloyed to its one side to produce single P-N junction. The single P-N junction justifies the name UJT.

36. What is negative resistance region in UJT? Explain its physical significance.

Ans. Once conduction is established at emitter voltage V_E = V_P, peak value of emitter voltage, the emitter potential V_E starts decreasing with the increase in emitter current I_E. This corresponds to a negative resistance region which is stable enough to be used with a great deal of reliability in the area of applications—oscillator.