Semiconductor

1. Semiconductors are:
   a) Perfect conductors
   b) Perfect insulators
   c) Materials with conductivity between conductors and insulators
   d) Superconductors
   Answer: c
   Explanation: Semiconductors conduct partially and fall between metals and insulators. (BPSC 65th)

2. Which of the following is a semiconductor?
   a) Copper
   b) Silver
   c) Silicon
   d) Glass
   Answer: c
   Explanation: Silicon is a commonly used intrinsic semiconductor. (BPSC Teacher 2023)

3. Which element is widely used in semiconductor devices?
   a) Aluminium
   b) Silicon
   c) Iron
   d) Lead
   Answer: b
   Explanation: Silicon has four valence electrons and forms stable crystals. (BPSC CDPO 2019)

4. Which of the following is an intrinsic semiconductor?
   a) Silicon
   b) Germanium
   c) Both a and b
   d) None
   Answer: c
   Explanation: Pure silicon and germanium are intrinsic semiconductors. (BPSC 64th)

5. In a P-type semiconductor, the majority charge carriers are:
   a) Electrons
   b) Holes
   c) Protons
   d) Neutrons
   Answer: b
   Explanation: P-type semiconductors have holes as majority carriers. (BPSC AE)

6. Which element is used to dope silicon for an N-type semiconductor?
   a) Boron
   b) Phosphorus
   c) Aluminium
   d) Gallium
   Answer: b
   Explanation: Phosphorus (Group 15) donates free electrons. (BPSC 67th)

7. What is doping in semiconductors?
   a) Adding impurities to increase resistance
   b) Adding impurities to improve conductivity
   c) Removing impurities
   d) None of the above
   Answer: b
   Explanation: Doping increases charge carriers, improving conductivity.

8. Which of the following devices is made from semiconductors?
   a) Resistor
   b) Capacitor
   c) Diode
   d) Inductor
   Answer: c
   Explanation: Diodes are made using P-N semiconductor junctions. (BPSC 66th)

9. A diode allows current to flow in:
   a) Both directions
   b) One direction only
   c) No direction
   d) All directions
   Answer: b
   Explanation: Diodes are unidirectional semiconductor devices.

10. Which device does not use semiconductors?
    a) Transistor
    b) IC
    c) Motor
    d) Diode
    Answer: c
    Explanation: Motors are electromechanical, not semiconductor-based.

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NCERT-Based Questions (Q11–30)

1. The energy band gap in semiconductors is:
   a) Very large
   b) Small
   c) Zero
   d) Infinite
   Answer: b
   Explanation: Semiconductors have a small band gap (~1 eV).

2. At absolute zero, semiconductors behave as:
   a) Metals
   b) Insulators
   c) Conductors
   d) Superconductors
   Answer: b
   Explanation: No free electrons exist at 0 K, so they act as insulators.

3. A material that becomes conductive after doping is a:
   a) Metal
   b) Alloy
   c) Semiconductor
   d) Insulator
   Answer: c
   Explanation: Doping makes intrinsic semiconductors conductive.

4. Which of the following is used in making transistors?
   a) Silicon
   b) Lead
   c) Aluminium
   d) Gold
   Answer: a
   Explanation: Silicon-based semiconductors are used in transistors.

5. Zener diode is mainly used for:
   a) Rectification
   b) Voltage regulation
   c) Current supply
   d) Filtration
   Answer: b
   Explanation: Zener diodes maintain constant voltage.

6. Solar cells work based on:
   a) Thermal conduction
   b) Magnetic field
   c) P-N junction
   d) Electrolysis
   Answer: c
   Explanation: Solar cells use semiconductor P-N junctions.

7. LED stands for:
   a) Light Energy Device
   b) Light Emitting Diode
   c) Luminous Electric Diode
   d) Linear Energy Diode
   Answer: b
   Explanation: LED is a diode that emits light when forward-biased.

8. Commonly used semiconductor in solar cells is:
   a) Gallium
   b) Aluminium
   c) Silicon
   d) Zinc
   Answer: c
   Explanation: Silicon is preferred for cost and efficiency.

9. P-type semiconductors are made by doping with:
   a) Pentavalent elements
   b) Trivalent elements
   c) Divalent elements
   d) Group 18 elements
   Answer: b
   Explanation: Trivalent atoms like Boron create holes.

10. Holes in semiconductors are:
    a) Negative charges
    b) Photons
    c) Positively charged vacancies
    d) Neutrons
    Answer: c
    Explanation: Holes are empty states that behave as positive carriers.

11. Forward biasing a P-N junction causes:
    a) No current
    b) Increased resistance
    c) Current to flow easily
    d) Junction breakdown
    Answer: c
    Explanation: Forward bias reduces potential barrier, allowing current.

12. Transistors are used for:
    a) Storing energy
    b) Generating AC
    c) Amplifying signals
    d) Breaking circuits
    Answer: c
    Explanation: Transistors amplify weak electrical signals.

13. Semiconductors are primarily used in:
    a) Generators
    b) Microprocessors
    c) Motors
    d) Switchboards
    Answer: b
    Explanation: Microchips are built using semiconductors.

14. Which of the following is a compound semiconductor?
    a) Silicon
    b) Germanium
    c) Gallium Arsenide
    d) Carbon
    Answer: c
    Explanation: GaAs is used in high-speed electronic devices.

15. Which semiconductor emits light?
    a) Transistor
    b) Zener diode
    c) LED
    d) Photodiode
    Answer: c
    Explanation: LEDs emit visible light.

16. The base region in a transistor is:
    a) Wide and highly doped
    b) Wide and lightly doped
    c) Thin and lightly doped
    d) Thin and highly doped
    Answer: c
    Explanation: Thin base allows efficient charge transfer.

17. Semiconductors are sensitive to:
    a) Magnetic field
    b) Humidity
    c) Temperature
    d) Air pressure
    Answer: c
    Explanation: Conductivity increases with temperature.

18. Which element is used to form P-type semiconductors?
    a) Phosphorus
    b) Boron
    c) Arsenic
    d) Sulphur
    Answer: b
    Explanation: Boron has three valence electrons (Group 13).

19. Which device converts light into electricity?
    a) LED
    b) Transistor
    c) Solar cell
    d) Rectifier
    Answer: c
    Explanation: Solar cells use the photovoltaic effect.

20. Compound semiconductor with higher speed than silicon:
    a) Germanium
    b) GaAs
    c) Boron
    d) Selenium
    Answer: b
    Explanation: GaAs has higher electron mobility than Si.

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Predicted Questions (Q31–50)

1. Majority carriers in N-type semiconductors are:
   a) Holes
   b) Electrons
   c) Ions
   d) Protons
   Answer: b
   Explanation: Extra electrons from donor impurity dominate.

2. Majority carriers in P-type semiconductors are:
   a) Protons
   b) Electrons
   c) Holes
   d) Neutrons
   Answer: c
   Explanation: Trivalent doping creates holes.

3. Which semiconductor device is used in logic circuits?
   a) Diode
   b) Transistor
   c) Capacitor
   d) Motor
   Answer: b
   Explanation: Transistors are core of logic gates and microprocessors.

4. In reverse bias, the P-N junction:
   a) Conducts heavily
   b) Breaks
   c) Does not conduct
   d) Amplifies current
   Answer: c
   Explanation: Reverse bias increases depletion layer, stopping current.

5. Which property makes semiconductors useful in computers?
   a) Flexibility
   b) Resistance
   c) Controllable conductivity
   d) High weight
   Answer: c
   Explanation: Logic gates use controllable on/off states.

6. When light falls on a photodiode, it:
   a) Emits light
   b) Absorbs light
   c) Produces current
   d) Vibrates
   Answer: c
   Explanation: Converts light energy to electrical energy.

7. Diode is used for:
   a) Modulation
   b) Amplification
   c) Rectification
   d) Ionization
   Answer: c
   Explanation: Diode converts AC to DC.

8. Which junction acts as a rectifier?
   a) Transistor
   b) Transformer
   c) P-N junction
   d) Zener
   Answer: c
   Explanation: P-N junction allows current in one direction.

9. Material used for high-frequency applications:
   a) Copper
   b) Iron
   c) GaAs
   d) Carbon
   Answer: c
   Explanation: Gallium Arsenide is used for microwave-frequency devices.

10. LEDs are made of:
    a) Conductors
    b) Superconductors
    c) Semiconductors
    d) Fluorescent materials
    Answer: c
    Explanation: Light-emitting diodes are P-N semiconductors.

11. What happens to resistance of semiconductor with rise in temp?
    a) Increases
    b) Decreases
    c) No change
    d) Becomes infinite
    Answer: b
    Explanation: More charge carriers are generated.

12. Doping with arsenic forms:
    a) P-type
    b) N-type
    c) Both
    d) None
    Answer: b
    Explanation: Arsenic is a donor (group 15).

13. What is a transistor made of?
    a) Resistor
    b) 3 P-N junctions
    c) 2 P-N junctions
    d) Single metal
    Answer: c
    Explanation: NPN or PNP configuration = 2 junctions.

14. Logic gates are built using:
    a) Capacitors
    b) Motors
    c) Transistors
    d) Inductors
    Answer: c
    Explanation: Logic circuits are based on transistor combinations.

15. Semiconductors used in smart cards:
    a) Gallium
    b) Silicon
    c) Sodium
    d) Iron
    Answer: b
    Explanation: Embedded circuits are made of silicon chips.

16. Electronic watches use:
    a) Magnet
    b) Mechanical coil
    c) Semiconductor IC
    d) Wood
    Answer: c
    Explanation: Integrated circuits based on semiconductors.

17. Semiconductors best conduct at:
    a) High temp
    b) Low temp
    c) Freezing point
    d) Absolute zero
    Answer: a
    Explanation: Higher temperature = more energy = better conduction.

18. Doping increases:
    a) Band gap
    b) Weight
    c) Charge carriers
    d) Melting point
    Answer: c
    Explanation: More electrons/holes are available.

19. Most commonly used semiconductor in India:
    a) Germanium
    b) Gallium
    c) Silicon
    d) Aluminium
    Answer: c
    Explanation: Silicon is abundant and economical.

20. In ICs, semiconductors replace:
    a) Wires
    b) Switches
    c) Mechanical parts
    d) Magnetic disks
    Answer: b
    Explanation: ICs perform logical switching using semiconductors.