Free Physics of Solids MCQs with Answers

11. When NPN transistor is used as an amplifier:

• A. Electrons move from base to collector
• B. Holes move from emitter to base
• C. Electrons move from collector to base
• D. Holes move from base to emitter

Explanation: In an NPN transistor used as an amplifier, the majority charge carriers are electrons, which flow from the N-type emitter into the P-type base and then proceed to the N-type collector. This movement is critical for the amplification process. Option A is correct because it describes the direction of electron flow correctly. Options B and D are incorrect as they describe the movement of holes, which are not the majority carriers in an NPN transistor. Option C is wrong because it incorrectly describes the direction of electron movement.

12. By increasing the temperature, the conductivity of the semiconductor is:

• A. Decreases
• B. Increases
• C. Constant
• D. Not affected

Explanation: In semiconductors, increasing the temperature provides additional energy to electrons, enabling more of them to break free and become charge carriers. This increase in free electrons or holes leads to an increase in conductivity. Therefore, option B is correct as it recognizes this increase in electrical conductivity with temperature.Option A is incorrect because it suggests a decrease in conductivity, which is opposite to what occurs in semiconductors. Option C is incorrect because conductivity does not remain constant with temperature changes. Option D is incorrect because it implies temperature has no effect on conductivity, which is not the case in semiconductors.

13. In biasing of a transistor, which region must be reverse biased?

• A. Emitter-base
• B. Collector-base
• C. Emitter-collector
• D. None of these

Explanation: The correct answer is Option B: Collector-base. In a transistor, the collector-base junction must be reverse biased to ensure that the majority of charge carriers are prevented from flowing from the base to the collector. This creates a high resistance barrier and is essential for the transistor to function properly. Option A is incorrect because the emitter-base junction needs to be forward biased to allow the flow of charge carriers. Option C is incorrect because there is no direct biasing of the emitter-collector junction in standard transistor operation. Option D is incorrect as it contradicts the requirement of reverse biasing the collector-base junction.

14. The pair production and intrinsic conductivity both are low at _.

• A. Road temperature
• B. Hall temperature
• C. Room temperature
• D. None of these

Explanation: At room temperature (usually around 20-25°C or 68-77°F), both pair production and intrinsic conductivity in semiconductors are relatively low compared to higher temperatures. While electrons can still gain enough energy to move and generate electron-hole pairs at room temperature, the thermal energy available is less compared to higher temperatures, resulting in a lower rate of pair production and intrinsic conductivity in semiconductors.

15. The germanium or silicon crystal formed after adding pentavalent impurity is known as _ substances.

• A. N-type
• B. S-type
• C. Q-type
• D. None of these

Explanation: We can gain an N-type semiconductor by adding pentavalent impurities such as phosphorus or arsenic. These elements have more electrons in their outer shells than elements like silicon or germanium, and when they're introduced as impurities into these semiconductors, they create an excess of free electrons, thereby giving rise to an N-type semiconductor.

16. Solar cell (or photo voltaic cell) is a device which converts:

• A. Chemical energy into electrical energy
• B. Light energy into electrical energy
• C. Electrical energy into light energy
• D. Chemical energy into light energy

Explanation: A solar cell, also known as a photovoltaic cell, is a device that converts light energy, typically from sunlight, directly into electrical energy through the photovoltaic effect.When photons (particles of light) strike the semiconductor material within a solar cell, they transfer their energy to the material. This energy excites electrons, allowing them to break free from their atomic bonds, resulting in the creation of electron-hole pairs. The internal electric field within the semiconductor then separates these charge carriers, generating an electric current.In summary, a solar cell or photovoltaic cell converts light energy into electrical energy by utilizing the photovoltaic effect within the semiconductor material.

17. Which of the following is NOT an electromagnetic wave?

• A. Complete valence band
• B. Partially filled conduction band
• C. Tightly bounded valence electrons
• D. Large energy gapped between electrons

Explanation: Electromagnetic waves are characterized by the oscillation of electric and magnetic fields propagating through space, such as visible light, radio waves, microwaves, and X-rays. Options A, B, and C do not describe the characteristics of electromagnetic waves, while Option D correctly identifies that electromagnetic waves do not have a large energy gap between electrons.

18. The substance(s) in which the atoms do NOT form magnetic dipole is/ are calledI. ferromagneticII. paramagneticIII. diamagnetic

• A. I only.
• B. III only.
• C. I and II.
• D. II and III.

Explanation: Diamagnetic substances are those in which the atoms do not possess a permanent magnetic dipole moment. When an external magnetic field is applied, a magnetic dipole moment is induced in the opposite direction of the applied field.

19. In terms of energy band, semiconductor materials at room temperature do NOT have

• A. partially filled valence band.
• B. partially filled conduction band
• C. plenty of free electrons for electrical conduction
• D. narrow forbidden energy gap between conduction and valence band

Explanation: Semiconductor materials at room temperature do not have a partially filled conduction band, as this would indicate metallic behavior. The correct answer is that they have a partially filled valence band, which is essential for their conductivity. The other options are incorrect because semiconductor materials do not have a narrow forbidden energy gap like insulators, nor do they have plenty of free electrons like metals.

20. Conductivity of a semiconductor material increases byI. increasing temperature.II. increasing the surface area.III. decreasing the density of charge carriers.IV. doping with small amounts of impurities.

• A. I and III.
• B. I and IV.
• C. II and III.
• D. II and IV.

Explanation: The correct answer is Option B: I and IV. Conductivity in semiconductors increases with temperature because thermal energy excites more electrons into the conduction band, thus increasing the number of charge carriers. Doping with small amounts of impurities introduces additional charge carriers (electrons or holes), which enhances conductivity.Option A is incorrect because although increasing temperature increases conductivity, decreasing the density of charge carriers would reduce conductivity. Option C is incorrect as increasing surface area does not directly affect conductivity, and decreasing the density of charge carriers would reduce it. Option D is incorrect because increasing surface area does not directly enhance conductivity, even though doping does.