Electronic Devices And Circuits — Part 1 MCQs with Explanations

Understanding Half-Wave Rectifiers: A half-wave rectifier only uses one-half of the input AC cycle to produce DC power. It essentially "chooses" the positive (or negative) part of the wave and discards the rest. Power Loss: Because it ignores half the input AC cycle, a significant portion of the input power is wasted as heat. This means the AC input power needed is much greater than the DC output power. Calculating Input Power: To find the AC input power, we need to consider this loss. For an ideal half-wave rectifier (no losses other than discarding half the wave), the relationship is approximately: AC Input Power ≈ 2 x DC Output Power Applying to the Problem: The question states that the DC output power is 500W. Therefore: AC Input Power ≈ 2 * 500W = 1000W Why the options are different: The options provided are slightly different from our simple calculation because real-world rectifiers have additional losses (like those in the transformer and diodes). However, our calculation gives us an approximation to help us pick the most likely answer. Considering the losses, the closest option is 1232 W. Therefore, the best option is A.

A thermistor is a special kind of resistor. Resistors are electronic components that resist the flow of electricity. A thermistor is unique because its resistance changes depending on its temperature. Think of it like this: a normal resistor always has the same resistance, no matter how hot or cold it is. But a thermistor's resistance goes up or down as its temperature changes. Option A is incorrect because a thermocouple measures temperature using the difference in voltage between two dissimilar metals. Option B is incorrect because a thermometer usually measures temperature using the expansion of a liquid or metal. Option C is the correct answer because a thermistor is basically a small resistor that's very sensitive to heat. Its resistance changes noticeably with temperature changes. Option D is incorrect because thermistors are not explosive. They are passive components.

Power diodes are used in applications that require handling high currents and voltages . Germanium diodes have a lower breakdown voltage compared to silicon diodes. This means they are not suitable for high-power applications where high voltages are involved. Silicon diodes, on the other hand, have a much higher breakdown voltage and can handle significantly larger currents. Therefore, power diodes are generally made from silicon because of its ability to withstand higher power levels. Hence, the correct answer is Option A: Silicon diodes .

Question: Why isn't silicon used to make light-emitting diodes (LEDs)? Explanation: LEDs work by emitting light when electrons fall from a higher energy level to a lower one. This happens most efficiently in materials with a direct band gap . Silicon is an indirect band gap semiconductor . This means that when an electron drops to a lower energy level, it doesn't directly emit a photon (light particle). Instead, it often loses energy through other processes (like vibrations in the crystal lattice) which don't produce light. Therefore, silicon isn't suitable for LEDs because its indirect band gap makes it inefficient at converting electrical energy into light . Materials like gallium arsenide (GaAs), which have a direct band gap, are much better choices for making LEDs. The correct answer is A: An indirect band gap semiconductor

Light-dependent resistors (LDRs) are special resistors whose resistance changes depending on the amount of light shining on them. To understand why, we need to know a bit about semiconductors. Semiconductors are materials that are neither good conductors (like metals) nor good insulators (like rubber). Their conductivity can be changed. Intrinsic semiconductors have a moderate number of charge carriers (electrons and holes) that allow them to conduct electricity, but this is relatively low. Lightly doped semiconductors have a small amount of impurities added to increase their conductivity. This makes them slightly better at conducting. Highly doped semiconductors have a large amount of impurities added, making them very good conductors. LDRs work because light creates more charge carriers in the semiconductor material. More carriers mean better conductivity, and thus lower resistance. To make this effect strong, LDRs are usually made from lightly doped semiconductors . The small amount of doping provides the basis for a significant change in resistance when light strikes the material. Therefore, the correct answer is C: Lightly doped semiconductor .

The question asks about the depletion layer width in a junction (like a diode). Think of the depletion layer as a region with very few free charge carriers. Let's look at each option: Option A: Decreases with light doping This is incorrect. Lighter doping means fewer charge carriers are available. To create the depletion region, more of the semiconductor needs to be depleted of its free carriers, so a lighter doping requires a *wider* depletion region. Option B: Is independent of applied voltage This is incorrect. The width of the depletion layer *definitely* changes with applied voltage. Applying a voltage will affect how easily charge carriers can move across the junction, and thus the size of the depletion region. Option C: Is increased under reverse bias This is correct. Reverse bias *widens* the depletion layer. Applying a reverse voltage pulls the charge carriers away from the junction, making the depletion region bigger and offering greater resistance to current flow. Option D: Increases with heavy doping This is incorrect. Heavier doping means more charge carriers are available. Only a *narrow* depletion region needs to be created to deplete the available carriers. Therefore, the answer is Option C .

In an intrinsic semiconductor like silicon, the number of free electrons is equal to the number of holes. This is because electrons are freed when a covalent bond is broken, creating both a free electron and a hole (a missing electron in the bond). As the temperature increases, more covalent bonds are broken due to increased thermal energy. This generates more electron-hole pairs. For every broken bond, one electron and one hole are created. Therefore, the number of free electrons and holes increases, and they increase by the same amount because they are generated in pairs. So, the correct answer is Option C.

The correct answer is D: Holes and electrons. Here's why: Intrinsic semiconductors are pure semiconductors, meaning they haven't been doped with any impurities. At room temperature , some of the electrons in the semiconductor gain enough energy to break free from their bonds. When an electron breaks free, it leaves behind a hole in the crystal structure. Both the free electrons and the holes can move through the material under the influence of an electric field. Therefore, the current in an intrinsic semiconductor at room temperature is due to the movement of both holes and electrons . Ions are not the primary charge carriers in intrinsic semiconductors.

In transistor configurations, different parameters are used to describe amplification properties. Forward amplification factor is denoted by α (alpha) and it represents the current gain in Common Base (CB) configuration . It is defined as the ratio of collector current to emitter current: α = Ic / Ie The value of α is always less than 1 but very close to it (typically between 0.95 and 0.99). Option A: Incorrect, because power gain of CE amplifier depends on both current and voltage gain, not on forward amplification factor. Option C: Incorrect, because CC amplifier has a voltage gain nearly equal to 1, which is not related to forward amplification factor. Option D: Incorrect, because voltage gain of CE amplifier is different and not defined by α. Thus, the forward amplification factor of a transistor corresponds to the current gain of CB amplifier .

A MOS capacitor consists of a metal gate, oxide layer, and semiconductor substrate. The behavior of the surface region depends on the type of substrate and the polarity of the applied gate voltage. For a P-type substrate with positive gate voltage: The applied positive potential repels holes (majority carriers) away from the surface and attracts electrons (minority carriers) towards the oxide–semiconductor interface. This leads to the formation of an N-type inversion layer . Hence, Option D is correct. Option A: Incorrect, because in an N-type substrate under negative voltage, holes move towards the surface causing accumulation, not inversion. Option B: Incorrect, because in a P-type substrate with negative voltage, holes are attracted to the surface, again causing accumulation, not inversion. Option C: Incorrect, because in an N-type substrate with positive voltage, electrons accumulate at the surface, leading to accumulation only, not inversion. Therefore, the correct statement is Option D .

Metals is the correct answer to “The forbidden energy gap between the valence band and conduction band will be least in case of”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Diffusion of holes is the correct answer to “For a NPN bipolar transistor, what is the main stream of current in the base region”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Both A and R are true and R is correct explanation of A is the correct answer to “Assertion (A): A JFET can be used as a current source. Reason (R): In beyond pinch off region the current in JFET is nearly constant.”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Rate of thermal generation of electron hole pairs is the correct answer to “For a P-N diode, the number of minority carriers crossing the junction depends on”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Has higher energy than the electron in the valence band is the correct answer to “An electron in the conduction band”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Reverse, voltage is the correct answer to “In an ideal diode there is no breakdown, no __________ current, and no forward __________ drop.”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Varies inversely with current is the correct answer to “The dynamic resistance of a forward biased p-n diode”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

6 MOS transistor is the correct answer to “Each cell of a static Random Access memory contains”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

False is the correct answer to “When avalanche breakdown occurs covalent bonds are not affected.”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Pure annealed copper is the correct answer to “Which variety of copper has the best conductivity”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Conduction band is the correct answer to “Free electrons exist in”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Only for the positive half cycle of the input signal is the correct answer to “In a half wave rectifier, the load current flows”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Both A and R are true but R is not a correct explanation of A is the correct answer to “Assertion (A): In p-n-p transistor collector current is termed negative. Reason (R): In p-n-p transistor holes are majority carriers.”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Only those produced by thermal energy is the correct answer to “How many free electrons does a p type semiconductor has”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Forward bias is the correct answer to “In a bipolar transistor, the emitter base junction has”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Excessive heat may damage the diode is the correct answer to “If too large current passes through the diode”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Air is the correct answer to “Which of the following has highest resistivity”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

100 is the correct answer to “A transistor has a current gain of 0.99 in the CB mode. Its current gain in the CC mode is”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Both A and R are true and R is correct explanation of A is the correct answer to “Assertion (A): A p-n junction has high resistance in reverse direction. Reason (R): When a reverse bias is applied to p-n junction, the width of depletion layer increases.”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Reverse bias is the correct answer to “In a bipolar transistor, the base collector junction has”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

False is the correct answer to “Work function is the maximum energy required by the fastest electron at 0 K to escape from the metal surface.”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

An ohmic region at low voltage value followed by a saturation region at higher voltages is the correct answer to “The output, V-I characteristics of an Enhancement type MOSFET has”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Both frequency and intensity of incident radiation is the correct answer to “The amount of photoelectric emission current depends on”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Green portion of spectrum is the correct answer to “The sensitivity of human eyes is maximum at”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

1 A is the correct answer to “Which of the following could be the maximum current rating of junction diode by 126”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Rectification of the signal is required is the correct answer to “Piezoelectric quartz crystal resonators find application where”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

200 mg is the correct answer to “In the sale of diamonds the unit of weight is carat. One carat is equal to”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

0.78 eV is the correct answer to “Forbidden energy gap in germanium at 0 K is about”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

A is true but R is false is the correct answer to “Assertion (A): A VMOS can handle much larger current than other field effect transistors. Reason (R): In a VMOS the conducting channel is very narrow.”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.

Majority carriers in both regions is the correct answer to “In a semiconductor diode, the barrier offers opposition to”. This question is standard in competitive examination syllabi for Electronic Devices And Circuits.