1. Semiconductor electronics mainly deals with materials and devices in which the flow of charge carriers is controlled. What is the most suitable description of this field?
ⓐ. Study of only metallic wires carrying current
ⓑ. Study of only insulating materials used for safety covers
ⓒ. Study of controlled semiconductor conduction
ⓓ. Study of mechanical switching without charge-carrier motion
Correct Answer: Study of controlled semiconductor conduction
Explanation: Semiconductor electronics is centred on materials whose electrical conductivity can be controlled in useful ways. Devices such as diodes, transistors, \(LED\)s, and solar cells work because charge carriers inside semiconductor materials can be guided by junctions, bias voltages, and material preparation. A metallic wire mainly offers conduction, but it does not provide the same controlled device action. An insulator is also not the central material here because it normally blocks charge motion. The key idea is controlled carrier motion, not simply the presence or absence of current.
2. A material has electrical conductivity greater than that of an insulator but smaller than that of a good conductor. The material is best classified as a:
ⓐ. semiconductor
ⓑ. superconductor
ⓒ. perfect insulator
ⓓ. metallic conductor
Correct Answer: semiconductor
Explanation: A semiconductor is defined by its intermediate conductivity range. Its conductivity is not as high as that of metals such as copper, but it is also not as low as that of strong insulators such as glass. This middle position allows its conduction to be changed by temperature, light, doping, or applied bias. That controllability is why semiconductors are used to make active devices. The classification is about electrical behaviour, not about whether the material looks metallic or non-metallic.
3. The pair that contains two common elemental semiconductors is:
ⓐ. copper and aluminium
ⓑ. glass and rubber
ⓒ. sodium and iron
ⓓ. silicon, germanium
Correct Answer: silicon, germanium
Explanation: Silicon and germanium are the standard elemental semiconductors introduced in basic semiconductor electronics. They have suitable band structures and form covalent crystals, making them useful for semiconductor devices. Copper, aluminium, sodium, and iron are metals with much higher conductivity. Glass and rubber are common insulators, so they do not represent the usual semiconductor material family. Silicon is especially important because of its stability and wide use in integrated circuits.
4. A mobile charger, an \(LED\) indicator, and a transistor-based switch all belong to semiconductor electronics because they:
ⓐ. work only by heating a metal filament
ⓑ. avoid electric current completely during operation
ⓒ. use controlled carrier motion in semiconductors
ⓓ. use only magnetic materials and no junctions
Correct Answer: use controlled carrier motion in semiconductors
Explanation: Many electronic circuits use semiconductor devices to control current and voltage in specific ways. An \(LED\) emits light when carriers recombine in a forward-biased semiconductor junction. A transistor can control a larger current using a smaller input signal. A charger circuit uses semiconductor diodes and switching devices to process electrical energy. The shared idea is not ordinary metallic conduction but controlled carrier action inside semiconductor materials.
5. A device is required to allow current mainly in one direction in a simple circuit. The most suitable semiconductor device for this basic role is a:
ⓐ. resistor only
ⓑ. diode
ⓒ. fuse wire
ⓓ. wooden switch
Correct Answer: diode
Explanation: A diode is a semiconductor device designed to conduct much more easily in one direction than in the opposite direction. This one-way conduction property makes it useful in rectifiers and protective circuits. A resistor does not normally provide one-way current behaviour; it mainly opposes current. A fuse wire is a safety device that melts under excessive current, and wood is an insulator. The diode example shows how semiconductor devices can perform functions that simple conductors cannot.
6. Study the entries in the table.
| Row | Item | Best classification |
|---|
| P | Silicon crystal used for device fabrication | Semiconductor material |
| Q | Copper connecting lead | Semiconductor material |
| R | Glass sleeve | Semiconductor device |
| S | Rubber coating | Semiconductor junction |
The row that gives a suitable classification is:
ⓐ. Row Q
ⓑ. Row R
ⓒ. Row S
ⓓ. Row P
Correct Answer: Row P
Explanation: Silicon is a common semiconductor material used in device fabrication. A copper lead is a metallic conductor, so it should not be classified as a semiconductor material. Glass and rubber are usually treated as insulators in basic circuit contexts. A semiconductor junction is formed inside semiconductor materials, not by a rubber coating or a glass sleeve. The table separates the material used for active electronic behaviour from ordinary conducting and insulating parts.
7. An electronic circuit uses a small signal to control a larger current in another part of the circuit. The semiconductor device family most closely connected with this action is:
ⓐ. fixed metal wire
ⓑ. ceramic insulator
ⓒ. transistor
ⓓ. dry wooden rod
Correct Answer: transistor
Explanation: A transistor is a semiconductor device that can control current and can also be used for amplification or switching. In a transistor circuit, a small input at one part of the device can influence a larger current in another part. A fixed metal wire can conduct current, but it does not provide this controlled action by itself. Ceramic and dry wood are usually insulating materials and are not used as active current-control devices. The word control is the clue that the device is more than a passive conducting path.
8. In a simple circuit, current and voltage are related to charge motion in different ways. The best description is:
ⓐ. Current is the cause of energy gap, while voltage is the unit of resistance
ⓑ. Current is charge-flow rate, while voltage is potential difference
ⓒ. Current is measured in \(\Omega\), while voltage is measured in \(A\)
ⓓ. Current and voltage are always the same physical quantity
Correct Answer: Current is charge-flow rate, while voltage is potential difference
Explanation: Electric current \(I\) describes how much charge flows per unit time through a circuit element. Voltage \(V\) represents potential difference and is connected with energy supplied or used per unit charge. These two quantities are related in circuits, but they are not the same physical quantity. Their units are also different: current is measured in \(A\), while voltage is measured in \(V\). Keeping \(I\) and \(V\) separate is essential before studying diode and transistor circuits.
9. The SI unit of electric current \(I\) is:
ⓐ. \(A\)
ⓑ. \(V\)
ⓒ. \(\Omega\)
ⓓ. \(eV\)
Correct Answer: \(A\)
Explanation: Electric current is represented by \(I\), and its SI unit is ampere, written as \(A\). The volt \(V\) is the unit of potential difference or voltage. The ohm \(\Omega\) is the unit of resistance. The electron volt \(eV\) is a unit of energy commonly used for energy gaps in semiconductor physics. Confusing \(A\) and \(V\) would mix up flow of charge with energy per unit charge.
10. In semiconductor band discussion, the forbidden energy gap \(E_g\) is commonly expressed in:
ⓐ. \(eV\)
ⓑ. \(A\)
ⓒ. \(\Omega\)
ⓓ. \(S\,m^{-1}\)
Correct Answer: \(eV\)
Explanation: The forbidden energy gap \(E_g\) is an energy difference between allowed energy bands. Because the energies involved in atoms and semiconductors are small on the joule scale, the unit \(eV\) is convenient. Ampere \(A\) is used for current, and ohm \(\Omega\) is used for resistance. Conductivity is measured in \(S\,m^{-1}\), not in \(eV\). The symbol \(E_g\) should be read as an energy quantity, so its unit must also be an energy unit.
11. A record shows \(R\), \(\rho\), and \(\sigma\) for a semiconductor sample. The notation is most suitably interpreted as:
ⓐ. \(R\) for resistivity, \(\rho\) for voltage, and \(\sigma\) for current
ⓑ. \(R\) for current, \(\rho\) for resistance, and \(\sigma\) for temperature
ⓒ. \(R\) for resistance, \(\rho\) for resistivity, and \(\sigma\) for conductivity
ⓓ. \(R\) for voltage, \(\rho\) for conductivity, and \(\sigma\) for energy gap
Correct Answer: \(R\) for resistance, \(\rho\) for resistivity, and \(\sigma\) for conductivity
Explanation: In basic semiconductor and circuit notation, \(R\) represents resistance. The symbol \(\rho\) represents resistivity, a material property that describes how strongly a material opposes current flow. The symbol \(\sigma\) represents conductivity, which describes how well a material conducts. These three are connected in meaning, but they are not interchangeable. The distinction between \(R\) and \(\rho\) is especially important because \(R\) belongs to a particular object, while \(\rho\) belongs to the material.
12. Match the quantities with their usual units.
| Column I | Column II |
|---|
| P. Resistance \(R\) | 1. \(S\,m^{-1}\) |
| Q. Conductivity \(\sigma\) | 2. \(\Omega\,m\) |
| R. Resistivity \(\rho\) | 3. \(\Omega\) |
| S. Voltage \(V\) | 4. \(V\) |
The suitable matching is:
ⓐ. P-1, Q-3, R-2, S-4
ⓑ. P-3, Q-2, R-1, S-4
ⓒ. P-3, Q-1, R-2, S-4
ⓓ. P-4, Q-1, R-2, S-3
Correct Answer: P-3, Q-1, R-2, S-4
Explanation: Resistance \(R\) is measured in ohm, written as \(\Omega\). Conductivity \(\sigma\) is measured in \(S\,m^{-1}\), where \(S\) stands for siemens. Resistivity \(\rho\) has unit \(\Omega\,m\). Voltage \(V\) is measured in volt, also written as \(V\). The similar appearance of the symbol \(V\) for voltage and the unit \(V\) for volt should not hide the fact that one is a physical quantity and the other is its unit.
13. A diode terminal list mentions anode and cathode. In the usual \(p\)-\(n\) diode naming convention:
ⓐ. anode is the \(n\)-side and cathode is the \(p\)-side
ⓑ. anode is the \(p\)-side and cathode is the \(n\)-side
ⓒ. both anode and cathode are always on the \(p\)-side
ⓓ. both anode and cathode are always on the \(n\)-side
Correct Answer: anode is the \(p\)-side and cathode is the \(n\)-side
Explanation: In a \(p\)-\(n\) junction diode, the terminal connected to the \(p\)-side is called the anode. The terminal connected to the \(n\)-side is called the cathode. These names are used later when describing forward bias and reverse bias. The terms do not mean that both terminals belong to the same side of the junction. Remembering the terminal names first makes circuit-polarity questions clearer later.
14. A three-terminal semiconductor device is labelled with emitter, base, and collector. This set of terminals belongs to a:
ⓐ. diode
ⓑ. BJT
ⓒ. resistor
ⓓ. capacitor
Correct Answer: BJT
Explanation: A transistor has three main terminals: emitter, base, and collector. A diode has two terminals, called anode and cathode. A resistor and a capacitor are also two-terminal circuit elements in the usual basic circuit representation. The presence of three named semiconductor terminals points directly to a transistor. This distinction prevents treating a transistor as just two ordinary diodes connected without transistor action.
15. A small quantity of charge has magnitude \(4.8\times10^{-19}\,\text{C}\). Taking the elementary charge as \(e=1.6\times10^{-19}\,\text{C}\), the number of elementary charges represented by this magnitude is:
ⓐ. \(3\)
ⓑ. \(2\)
ⓒ. \(4\)
ⓓ. \(6\)
Correct Answer: \(3\)
Explanation: \( \textbf{Given charge:} \) \(q=4.8\times10^{-19}\,\text{C}\).
\( \textbf{Elementary charge:} \) \(e=1.6\times10^{-19}\,\text{C}\).
\( \textbf{Required quantity:} \) Number of elementary charge units \(n\).
\( \textbf{Useful relation:} \)
\[
q=ne
\]
\( \textbf{Why this relation applies:} \) Charge is measured in multiples of the elementary charge magnitude.
\( \textbf{Rearrangement:} \)
\[
n=\frac{q}{e}
\]
\( \textbf{Substitution:} \)
\[
n=\frac{4.8\times10^{-19}}{1.6\times10^{-19}}
\]
\( \textbf{Calculation:} \)
\[
n=\frac{4.8}{1.6}=3
\]
\( \textbf{Final answer:} \) The charge magnitude represents \(3\) elementary charge units.
16. The unit pair most suitable for \(\rho\) and \(\sigma\) is:
ⓐ. \(\rho\) in \(S\,m^{-1}\), \(\sigma\) in \(\Omega\,m\)
ⓑ. \(\rho\) in \(A\), \(\sigma\) in \(V\)
ⓒ. \(\rho\) in \(eV\), \(\sigma\) in \(K\)
ⓓ. \(\rho\) in \(\Omega\,m\), \(\sigma\) in \(S\,m^{-1}\)
Correct Answer: \(\rho\) in \(\Omega\,m\), \(\sigma\) in \(S\,m^{-1}\)
Explanation: Resistivity is represented by \(\rho\), and its unit is \(\Omega\,m\). Conductivity is represented by \(\sigma\), and its unit is \(S\,m^{-1}\). These units show opposite types of material behaviour: resistivity measures opposition to conduction, while conductivity measures ease of conduction. Ampere \(A\) and volt \(V\) are circuit quantity units, not material-property units here. The pair \(\rho\) with \(\Omega\,m\) and \(\sigma\) with \(S\,m^{-1}\) keeps the material properties correctly separated.
17. A note in a circuit file says: “Apply a bias voltage across the semiconductor device before observing its current.” Here, bias voltage means:
ⓐ. the fixed atomic charge \(e\) inside every atom
ⓑ. the unit used to measure forbidden energy gap \(E_g\)
ⓒ. the resistance of the connecting wire only
ⓓ. an applied voltage used to set device operation
Correct Answer: an applied voltage used to set device operation
Explanation: Bias voltage is an externally applied voltage used to place a semiconductor device in a particular operating condition. In diode and transistor circuits, the polarity and magnitude of bias decide how carriers move through junctions. It is not the elementary charge \(e\), because \(e\) is a charge magnitude. It is also not the unit \(eV\), which is used for energy. The word bias points to circuit operation, not to the material’s energy-gap unit.
18. Consider the following statements.
I. \(E_g\) denotes forbidden energy gap.
II. Temperature is commonly represented by \(T\).
III. The \(p\)-side of a diode is called the cathode.
The valid set is:
ⓐ. I and II only
ⓑ. I only
ⓒ. II and III only
ⓓ. I, II, and III
Correct Answer: I and II only
Explanation: The symbol \(E_g\) is used for the forbidden energy gap in semiconductor band discussion. Temperature is commonly represented by \(T\), and temperature changes are important in semiconductor conductivity. The third statement is not valid because the \(p\)-side of a diode is called the anode, while the \(n\)-side is called the cathode. This terminal naming is basic before studying diode biasing. The pair \(E_g\) and \(T\) belongs to material behaviour, while anode and cathode belong to diode terminals.
19. In an energy-band description of a solid, the valence band mainly refers to:
ⓐ. the empty space outside the crystal where no electron can exist
ⓑ. the band of bonding valence electrons
ⓒ. the band used only for nuclear energy levels
ⓓ. the external battery terminal connected to the solid
Correct Answer: the band of bonding valence electrons
Explanation: The valence band is the allowed energy band that contains valence electrons involved in bonding. In a semiconductor or insulator at low temperature, this band is usually filled or nearly filled. Electrons in this band are not as free to move through the crystal as conduction electrons. The conduction band is the higher allowed band where electrons can contribute more directly to electrical conduction. The name valence band should be connected with bonding electrons, not with an external circuit terminal.
20. The conduction band of a solid is important because electrons in this band:
ⓐ. are always fixed permanently in covalent bonds
ⓑ. are nuclear particles inside the nucleus
ⓒ. cannot move even when an electric field is applied
ⓓ. can take part in electrical conduction
Correct Answer: can take part in electrical conduction
Explanation: The conduction band is an allowed energy band in which electrons can move more freely through the crystal. When electrons reach this band, they can respond to an applied electric field and help produce current. In a semiconductor, only a limited number of electrons reach the conduction band at ordinary temperature, so the conductivity is moderate. In a metal, electrons are already available for conduction in large numbers. The conduction band idea explains current in terms of electron energy states inside the solid.
