Semiconductor physics

Practice the basics of semiconductor physics — band gap, carriers, donors and acceptors — with multiple-choice questions and explanations.

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Semiconductor physics: key points

The properties of a semiconductor are largely determined by its band gap, the energy difference between the valence band and the conduction band. Silicon's band gap is about 1.12 eV at room temperature: not as large as an insulator's, and not zero like a metal's.

Adding impurities produces n-type semiconductors with many electrons or p-type semiconductors with many holes. The temperature dependence of carrier density, mobility, and the difference between drift and diffusion currents form the foundation for understanding the devices you will study later.

All Semiconductor physics questions (50)

  1. Which value is closest to the band gap of silicon (Si) at room temperature?
  2. Which statement is correct when phosphorus (P) is added to silicon as an impurity?
  3. What happens to the intrinsic carrier density when the temperature of an intrinsic semiconductor is raised?
  4. Which correctly compares electron and hole mobility in silicon?
  5. Where is the Fermi level located in an n-type semiconductor?
  6. Which correctly describes how the resistivity of an intrinsic semiconductor depends on temperature?
  7. In a semiconductor in thermal equilibrium, what is the product of electron density n and hole density p (n×p)? (ni: intrinsic carrier density)
  8. What causes a diffusion current to flow in a semiconductor?
  9. Which is a typical factor that shortens the minority carrier lifetime in silicon?
  10. What is the crystal structure of single-crystal silicon?
  11. What is the drift velocity of electrons with a mobility of 1000 cm²/V·s in an electric field of 100 V/cm?
  12. What is the mass introduced so that the motion of an electron in a crystal can be treated like that of a free electron in response to external forces?
  13. What does the Fermi level mean?
  14. What is the approximate hole density in n-type silicon with a donor concentration of 1×10¹⁶ cm⁻³ (all donors ionized, intrinsic carrier density ni = 1×10¹⁰ cm⁻³)?
  15. Which light can silicon absorb?
  16. What type of band structure does silicon have?
  17. What can be learned from a Hall effect measurement?
  18. What is a semiconductor called whose impurity concentration is so high that the Fermi level lies inside the conduction or valence band?
  19. Which is the Einstein relation between the diffusion coefficient D and the mobility μ of carriers? (k: Boltzmann constant, T: absolute temperature, q: elementary charge)
  20. Which value is closest to the thermal voltage kT/q at room temperature (300 K)?
  21. Where in the band gap is the donor level created by phosphorus in silicon?
  22. Silicon contains both donors at 5×10¹⁶ cm⁻³ and acceptors at 2×10¹⁶ cm⁻³ (room temperature, all ionized). Which describes it correctly?
  23. Why does the carrier density of n-type silicon drop sharply when it is cooled to cryogenic temperatures?
  24. Which recombination process becomes significant at high carrier concentrations, where the released energy is given to another carrier instead of being emitted as light?
  25. What is the approximate resistivity of n-type silicon with an electron density of 1×10¹⁶ cm⁻³ and an electron mobility of 1000 cm²/V·s? (Ignore holes; elementary charge 1.6×10⁻¹⁹ C)
  26. What is the energy difference between the vacuum level and the bottom of the conduction band of a semiconductor called?
  27. In lightly doped silicon, what is the main reason mobility decreases as temperature rises?
  28. Which value is closest to the intrinsic carrier density of silicon at room temperature?
  29. Which value is closest to the atomic density of crystalline silicon?
  30. Which correctly describes a hole?
  31. Which value is closest to the band gap of germanium (Ge) at room temperature?
  32. How many valence (outermost-shell) electrons does a silicon atom have?
  33. Which statement is correct when boron (B) is added to silicon as an impurity?
  34. What are the majority carriers in a p-type semiconductor?
  35. Which of the following materials has the largest band gap?
  36. Which is the correct unit of carrier mobility?
  37. With electron density n, hole density p, electron mobility μn, hole mobility μp and elementary charge q, which expression gives the conductivity σ of a semiconductor?
  38. What is the process called in which electrons and holes recombine through deep levels (traps) in the band gap?
  39. With minority carrier diffusion coefficient D and lifetime τ, which expression gives the diffusion length L?
  40. How does the band gap of silicon change as the temperature rises?
  41. What happens to the drift velocity of electrons in silicon when a strong electric field is applied?
  42. Which value is closest to the energy of a single photon of light with a wavelength of 620 nm? (E [eV] ≈ 1240 / λ [nm])
  43. How does carrier mobility generally change when the impurity concentration in silicon is increased?
  44. Which is the correct definition of the work function of a material?
  45. Which value is closest to the relative permittivity of silicon?
  46. Which value is closest to the electron mobility in lightly doped silicon at room temperature?
  47. In an indirect-gap semiconductor such as silicon, why is the probability of electron-hole recombination with light emission low?
  48. In a semiconductor driven out of thermal equilibrium by illumination or current injection, what are the levels introduced to describe the electron and hole densities separately?
  49. Which value is closest to the lattice constant of crystalline silicon?
  50. Why is the intrinsic Fermi level of silicon slightly offset from the exact center of the band gap?

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