Power Semiconductor Devices Questions and Answers

Q: TRIAC is a bidirectional power electronic device which contains :

A TRIAC contains two SCRs connected in reverse parallel.A TRIAC behaves like two thyristors connected in inverse parallel (anti-parallel) with a common gate, built on one chip. One SCR conducts in the positive half cycle and the other in the negative half cycle, so the device can switch current in both directions, which is why it is used for AC power control such as light dimmers and fan regulators.Two SCRs in series would conduct in one direction only. Two diodes in series cannot be triggered by a gate, and BJTs in parallel do not have the latching action of a thyristor.

Q: Field breakdown voltage for silicon used in power devices is :

The breakdown field of silicon is about 300 kV/cm.The critical electric field for avalanche breakdown in silicon is about \(3 \times 10^{5}\) V/cm, depending somewhat on doping. This figure sets how thick the drift region of a power device must be for a given blocking voltage, and is why high-voltage silicon devices need wide, lightly doped regions.100 kV/cm is too low. Around 1 MV/cm is not silicon's figure, and about 10 MV/cm is the breakdown field of SiO2, the insulator, not of silicon.

Direction for [ Question No: 1 To 1 ] :

Directions : In the following questions, match List - I and List - II and select the correct answer using the codes given below the lists :

List - I	List - II
a. relaxation oscillator	i. PWM
b. S.M.P.S.	ii. triac
c. population inversion	iii. UJT
d. bi-directional control	iv. Laser

Codes : a b c d

A.iii i iv ii
B.iii ii i iv
C.iv i ii iii
D.iii i ii iv

Exam Year(s)

UGC NET - 2012,

Answer: A

The matching is: a relaxation oscillator with UJT (iii), b S.M.P.S. with PWM (i), c population inversion with laser (iv), d bi-directional control with triac (ii), the code iii i iv ii (option A).

A UJT is the classic relaxation oscillator device, a switched-mode power supply regulates by pulse-width modulation, population inversion is the condition for laser action, and a triac conducts in both directions. Options B, C and D pair the triac or the laser with the wrong item.

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Which of the following is the symbol of zener diode?

Exam Year(s)

DSSSB - 2023,

Answer: C

The symbol of a zener diode is the one with the bent bar at the cathode, shown as (C). In the symbol, the triangle shows the direction of forward current and the bar at the cathode has its ends turned in the shape of a Z to show the breakdown action.

Symbol (A) is a light-emitting diode with arrows leaving the diode, (B) is a photodiode with arrows going towards it, and (D) is an ordinary junction diode with a straight bar. So the third option is the answer.

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TRIAC is a bidirectional power electronic device which contains :

A.Two BJTs connected in parallel
B.Two pn junction diodes connected in series
C.Two SCRs connected in reverse parallel
D.Two SCRs connected in series

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

A TRIAC contains two SCRs connected in reverse parallel.

A TRIAC behaves like two thyristors connected in inverse parallel (anti-parallel) with a common gate, built on one chip. One SCR conducts in the positive half cycle and the other in the negative half cycle, so the device can switch current in both directions, which is why it is used for AC power control such as light dimmers and fan regulators.

Two SCRs in series would conduct in one direction only. Two diodes in series cannot be triggered by a gate, and BJTs in parallel do not have the latching action of a thyristor.

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Field breakdown voltage for silicon used in power devices is :

A.100 kV/cm
B.300 kV/cm
C.1 MV/cm
D.10 MV/cm

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

The breakdown field of silicon is about 300 kV/cm.

The critical electric field for avalanche breakdown in silicon is about \(3 \times 10^{5}\) V/cm, depending somewhat on doping. This figure sets how thick the drift region of a power device must be for a given blocking voltage, and is why high-voltage silicon devices need wide, lightly doped regions.

100 kV/cm is too low. Around 1 MV/cm is not silicon's figure, and about 10 MV/cm is the breakdown field of SiO₂, the insulator, not of silicon.

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Match the following for on-state resistance for the device in milliohms :

Set I	Set II
(a) Diode	(1) 0.25
(b) BJT	(2) 0.16
(c) SCR	(3) 3.5
(d) Triac	(4) 4.0

Codes : (a) (b) (c) (d)

A.(2) (4) (1) (3)
B.(3) (4) (1) (2)
C.(4) (2) (3) (1)
D.(1) (2) (4) (3)

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: A

The key pairs diode with 0.16, BJT with 4.0, SCR with 0.25 and triac with 3.5 milliohms, which is code (2) (4) (1) (3).

This is a recall item with no formula behind it: the on-state resistance of a power device depends on its rating, voltage class and technology, so the values are only meaningful for the particular devices in whatever table the paper's author used. I could not trace that table to a published source, so I could not confirm the four pairings independently, and nothing I found shows them to be wrong. The key is therefore kept as the University printed it.

Do not read the order of these four numbers as a general ranking of device types: other ratings of the same devices give different resistances.

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Power Semiconductor Devices Questions and Answers

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