Ch-14: Semiconductor Electronics: Materials, Devices and Simple Circuits20 Questions
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1. In the band theory of solids, electrons that participate in electrical conduction occupy the:
A. Valence band only, always
B. Conduction band
C. Forbidden energy gap
D. Nucleus
Answer: B. Conduction band
Explanation: Electrons that are free to move and contribute to electrical conduction in a solid occupy energy states in the conduction band, above the forbidden energy gap.
2. The main difference between conductors, semiconductors, and insulators lies in:
A. Their color
B. The width of the forbidden energy gap between the valence and conduction bands
C. The number of protons in their atoms
D. Their melting points only
Answer: B. The width of the forbidden energy gap between the valence and conduction bands
Explanation: Conductors have overlapping or no energy gap, semiconductors have a small energy gap (~1 eV), and insulators have a large energy gap (>3 eV) between valence and conduction bands, determining their conductivity.
3. An intrinsic semiconductor is one that is:
A. Heavily doped with impurities
B. Pure, without any deliberately added impurities
C. Always a good conductor like a metal
D. Only found in liquid form
Answer: B. Pure, without any deliberately added impurities
Explanation: An intrinsic semiconductor is a pure semiconductor material, such as pure silicon or germanium, with no significant intentional impurity doping.
4. An extrinsic semiconductor is obtained by:
A. Cooling a pure semiconductor to absolute zero
B. Deliberately adding a small, controlled amount of impurity atoms to a pure semiconductor
C. Melting the semiconductor
D. Removing all electrons from the material
Answer: B. Deliberately adding a small, controlled amount of impurity atoms to a pure semiconductor
Explanation: Extrinsic semiconductors are created by doping — intentionally introducing a controlled amount of specific impurity atoms into a pure (intrinsic) semiconductor to modify its conductivity.
5. An n-type semiconductor is formed by doping a pure semiconductor (like silicon) with:
A. Trivalent impurity atoms (e.g., boron)
B. Pentavalent impurity atoms (e.g., phosphorus, arsenic)
C. Pure carbon
D. No impurities at all
Answer: B. Pentavalent impurity atoms (e.g., phosphorus, arsenic)
Explanation: Doping silicon with a pentavalent element, which has five valence electrons, contributes an extra free electron per atom, creating an n-type semiconductor with electrons as majority charge carriers.
6. A p-type semiconductor is formed by doping a pure semiconductor with:
A. Pentavalent impurity atoms
B. Trivalent impurity atoms (e.g., boron, indium), creating holes
C. Only tetravalent atoms
D. Metals like copper
Answer: B. Trivalent impurity atoms (e.g., boron, indium), creating holes
Explanation: Doping with a trivalent element, having only three valence electrons, creates a deficiency of an electron (a 'hole') at each impurity site, resulting in a p-type semiconductor where holes are majority carriers.
7. In a p-n junction diode, the region near the junction depleted of free charge carriers is called the:
A. Conduction band
B. Depletion region
C. Valence region
D. Forbidden zone
Answer: B. Depletion region
Explanation: Near the p-n junction, diffusion of electrons and holes across the boundary leaves behind a narrow region devoid of mobile charge carriers, known as the depletion region or depletion layer.
8. A p-n junction diode conducts significant current when it is:
A. Forward biased, with p-side connected to positive terminal and n-side to negative terminal
B. Reverse biased always
C. Not connected to any voltage source
D. Connected in either direction equally
Answer: A. Forward biased, with p-side connected to positive terminal and n-side to negative terminal
Explanation: In forward bias, connecting the p-side to the positive terminal and n-side to the negative terminal narrows the depletion region and allows significant current to flow through the diode.
9. A p-n junction diode is commonly used as a:
A. Amplifier
B. Rectifier, to convert AC into DC
C. Oscillator only
D. Light source in all cases
Answer: B. Rectifier, to convert AC into DC
Explanation: Since a diode conducts current mainly in one direction, it is widely used as a rectifier to convert alternating current (AC) into pulsating direct current (DC).
10. A full-wave rectifier, compared to a half-wave rectifier, utilizes:
A. Only the positive half cycle of AC input
B. Both the positive and negative half cycles of the AC input
C. Neither half cycle
D. Only DC input
Answer: B. Both the positive and negative half cycles of the AC input
Explanation: A full-wave rectifier is designed to convert both the positive and negative half cycles of an alternating input into a unidirectional (DC) output, making it more efficient than a half-wave rectifier.
11. A Zener diode is a specially designed p-n junction diode primarily used for:
A. Light emission
B. Voltage regulation, by operating in the reverse breakdown region
C. Amplifying audio signals
D. Storing electrical charge
Answer: B. Voltage regulation, by operating in the reverse breakdown region
Explanation: A Zener diode is designed to operate safely in reverse breakdown at a fixed voltage, making it ideal for use as a voltage regulator that maintains a constant output voltage.
12. A Light Emitting Diode (LED) converts electrical energy into light energy when it is:
A. Reverse biased
B. Forward biased, causing recombination of electrons and holes that releases energy as photons
C. Not connected to any circuit
D. Cooled to very low temperatures
Answer: B. Forward biased, causing recombination of electrons and holes that releases energy as photons
Explanation: When forward biased, electrons and holes recombine at the junction of an LED, and the released energy is emitted in the form of light (photons), rather than heat as in ordinary diodes.
13. A photodiode is generally operated in:
A. Forward bias, to emit light
B. Reverse bias, so its reverse current changes measurably with incident light intensity
C. No bias condition only
D. AC bias only
Answer: B. Reverse bias, so its reverse current changes measurably with incident light intensity
Explanation: A photodiode is normally operated under reverse bias; the small reverse saturation current increases in proportion to the intensity of light falling on the junction, allowing it to detect light.
14. A solar cell operates by converting light energy into electrical energy, and it is typically used:
A. With an external bias voltage applied
B. Without any external bias, generating its own voltage due to the photovoltaic effect
C. Only in reverse bias mode
D. Only as a rectifier
Answer: B. Without any external bias, generating its own voltage due to the photovoltaic effect
Explanation: A solar cell generates its own electromotive force via the photovoltaic effect when light falls on the junction, and it is operated without any externally applied bias voltage.
15. A junction transistor has three doped regions known as the:
A. Anode, cathode, and grid
B. Emitter, base, and collector
C. Source, gate, and drain
D. Positive, negative, and neutral terminals
Answer: B. Emitter, base, and collector
Explanation: A bipolar junction transistor consists of three regions: the emitter (which supplies charge carriers), the base (a thin middle region), and the collector (which collects the charge carriers).
16. In a common-emitter transistor amplifier configuration, a small change in base current produces:
A. No change in collector current
B. A proportionally larger change in collector current, providing current amplification
C. A decrease in collector current
D. A change only in emitter voltage
Answer: B. A proportionally larger change in collector current, providing current amplification
Explanation: In common-emitter configuration, the transistor amplifies a small base current into a much larger collector current change, characterized by the current gain factor β, making it useful as an amplifier.
17. In a transistor, the relationship between emitter current (IE), base current (IB), and collector current (IC) is:
A. IE = IB + IC
B. IE = IB − IC
C. IC = IB + IE
D. IB = IE + IC
Answer: A. IE = IB + IC
Explanation: By Kirchhoff's current law applied to a transistor, the total emitter current equals the sum of the base current and the collector current, i.e., IE = IB + IC.
18. When used as a switch, a transistor is operated in the:
A. Active region only
B. Saturation region (fully ON) and cutoff region (fully OFF)
C. Breakdown region only
D. Linear amplification region only
Answer: B. Saturation region (fully ON) and cutoff region (fully OFF)
Explanation: As an electronic switch, a transistor is driven between saturation (fully conducting, acting as a closed switch) and cutoff (no conduction, acting as an open switch), rather than the linear active region used for amplification.
19. A logic gate that gives a HIGH (1) output only when all its inputs are HIGH (1) is called a/an:
A. OR gate
B. AND gate
C. NOT gate
D. NOR gate
Answer: B. AND gate
Explanation: An AND gate produces a logical HIGH output only if every one of its inputs is HIGH; if even one input is LOW, the output remains LOW.
20. Which of the following logic gates is known as a 'universal gate' because any other logic gate can be constructed using only it?
A. OR gate
B. AND gate
C. NAND gate
D. XOR gate
Answer: C. NAND gate
Explanation: NAND (and also NOR) gates are called universal gates because any Boolean logic function, including AND, OR, and NOT operations, can be implemented using only combinations of NAND gates.
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