Energy Bands - Conductor, Insulator, Semiconductor - UNSOLVED PRACTICE SET
Chapter: Semiconductor Electronics | Topic: Energy Bands Conductor Insulator Semiconductor
ENERGY BANDS - CONDUCTOR, INSULATOR, SEMICONDUCTOR - UNSOLVED PRACTICE SET
Topic: Energy Bands Conductor Insulator Semiconductor
Multiple Choice Questions
Q1. In a solid, the energy band that contains electrons participating in electrical conduction is called:
- Valence band
- Conduction band
- Forbidden band
- Fermi level
Q2. The energy gap between the valence band and conduction band in an insulator is typically:
- Less than 1 eV
- Between 1 eV and 3 eV
- Greater than 3 eV
- Zero
Q3. In a conductor at room temperature:
- The valence band is completely filled and the conduction band is empty
- The valence band and conduction band overlap
- There is a large forbidden gap between the valence and conduction bands
- Electrons cannot move freely
Q4. The energy band gap of silicon at room temperature is approximately:
- 0.67 eV
- 1.1 eV
- 5.5 eV
- 6.5 eV
Q5. At absolute zero temperature (0 K), a semiconductor behaves like:
- A conductor
- An insulator
- A superconductor
- A perfect conductor
Q6. The Fermi level in an intrinsic semiconductor at room temperature lies:
- At the top of the valence band
- At the bottom of the conduction band
- In the middle of the forbidden gap
- Outside the energy bands
Short Answer Questions
Q7. What are energy bands in a solid? Distinguish between valence band and conduction band.
Q8. Why does a conductor have a large number of free electrons at room temperature, while an insulator has almost none?
Q9. Explain why the electrical conductivity of a semiconductor increases with temperature.
Q10. Compare the energy band diagrams of a conductor, an insulator, and a semiconductor at room temperature.
Q11. What is the significance of the band gap in determining the electrical properties of a material?
Q12. Why does diamond (band gap ~ 5.5 eV) behave as an insulator while silicon (band gap ~ 1.1 eV) behaves as a semiconductor?
Long Answer Questions
Q13. Explain the formation of energy bands in solids using the concept of atomic energy levels. Describe how the energy band structure determines whether a material is a conductor, insulator, or semiconductor.
Q14. Draw the energy band diagrams for a conductor, an insulator, and a semiconductor at room temperature. Explain the position of the Fermi level in each case and how it affects electrical conductivity.
Q15. Discuss the effect of temperature on the energy band structure and electrical conductivity of semiconductors, conductors, and insulators. Why does the conductivity of a semiconductor increase while that of a conductor decreases with increasing temperature?
Numerical & Application-based Problems
Q16. The band gap of germanium is 0.67 eV and that of silicon is 1.1 eV at room temperature (300 K). Boltzmann constant k = 8.6 ร 10โปโต eV/K.
(a) Calculate the thermal energy (kT) of electrons at room temperature in eV.
(b) Compare this thermal energy with the band gaps of germanium and silicon.
(c) Explain why this comparison helps us understand why germanium has higher intrinsic carrier concentration than silicon at room temperature.
Q17. The band gap of a semiconductor decreases with temperature according to the relation E_g(T) = E_g(0) โ ฮฑT, where for silicon E_g(0) = 1.17 eV and ฮฑ = 4.73 ร 10โปโด eV/K.
(a) Calculate the band gap of silicon at 300 K.
(b) Calculate the band gap of silicon at 500 K.
(c) Explain how the decrease in band gap with temperature affects the conductivity of the semiconductor.
Q18. In your school science fair, a student creates a display comparing different materials using energy band concepts.
(a) She lists copper as a conductor, glass as an insulator, and silicon as a semiconductor. Draw the energy band diagrams for these three materials and explain why each behaves differently when a voltage is applied.
(b) The student explains that LED lights use semiconductors like gallium arsenide (band gap ~ 1.4 eV). Calculate the approximate wavelength of light emitted by this LED and identify its colour.
(c) She then discusses why blue LEDs (using gallium nitride, band gap ~ 3.4 eV) were much harder to develop than red LEDs. Explain the connection between band gap and the difficulty of creating reliable semiconductor devices.
(d) A classmate asks why we can't make a semiconductor with a band gap of 0.1 eV that works at room temperature. Explain the problems this would cause for device operation.
(e) In India, solar cell manufacturing is growing rapidly. Explain why silicon (band gap 1.1 eV) is preferred over germanium (band gap 0.67 eV) for solar cells, despite germanium having a smaller band gap.