Power Semiconductor Devices Questions and Answers

Answer: A

A triac can be considered as two SCRs connected in antiparallel with a common gate, which is option (A). Each SCR conducts during one half cycle, so together they control ac power in both directions from a single gate terminal. Two SCRs in parallel (B) would conduct in the same direction only. Two resistors (C) are not switching devices, and two GTOs with a common cathode gate (D) describes a different arrangement.

Answer: A

A diac is a two terminal bidirectional switch, which is option (A). The passage calls the triac a diac with a gate terminal, which means the diac has two terminals and no gate. It breaks over and conducts in both polarities of applied voltage, so it is bidirectional. Options (C) and (D) give it a third terminal, and (B) makes it unilateral, which describes a different device.

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.

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.

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.

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.

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.

Answer: A

The intrinsic stand-off ratio of a UJT is \(\eta = \dfrac{R_{B1}}{R_{BB}}\), where the interbase resistance is \(R_{BB} = R_{B1} + R_{B2}\). With the given values,

\[R_{B1} = 10 - 4 = 6\ \text{k}\Omega\]

\[\eta = \dfrac{6}{10} = 0.6\]

This is the first option. The value 0.4 is the ratio \(R_{B2}/R_{BB}\), which is taken by mistake, and 2.4 and 3.5 are greater than one, which is impossible for a ratio of part of a resistance to the whole.

Answer: A

Both statements are true and the reason explains the assertion, so the answer is option (A).

A UJT is used in relaxation oscillators. Once the emitter voltage reaches the peak point, the device enters a negative resistance region and discharges the timing capacitor quickly, which gives the repeated charge and discharge cycle.

That negative resistance region is the reason given, so (B) is wrong. Options (C) and (D) call a true statement false.

Answer: A

A unijunction transistor (UJT) has a negative resistance region: once the emitter voltage reaches the peak point, the emitter-to-base-1 resistance collapses, the emitter voltage falls as the current rises, and this negative slope is what lets a UJT work as a relaxation oscillator. It is not a bipolar device, because conduction is by majority carriers in a single bar of n-type silicon with one p-n junction. It has a high, not low, firing current compared with a thyristor trigger, and 'relax indefinitely' has no meaning here. The answer is negative resistance characteristics.