Semiconductors Knowledge-Based Download PDF

Questions

1. 1.

   Which statement correctly describes the localized electronic energy state introduced into the band structure when crystalline silicon is doped with boron?

   • A) A donor level is formed just below the conduction band, allowing electrons to be easily promoted into the conduction band.
   • B) An acceptor level is formed deep within the center of the band gap, promoting non-radiative recombination of charge carriers.
   • C) An acceptor level is formed just above the valence band, allowing valence electrons to be thermally excited into it and leaving mobile holes behind.
   • D) A donor level is formed just above the valence band, directly injecting mobile positive ions into the crystal lattice.
2. 2.

   At the interface of an unbiased p–n junction, what physical mechanism is directly responsible for the formation of the depletion region and the associated built-in potential barrier?

   • A) Drift of minority charge carriers driven by externally applied thermal gradients across the interface
   • B) Mutual diffusion of majority carriers across the interface and subsequent recombination, leaving behind immobile ionized dopant atoms
   • C) Direct quantum tunneling of valence band electrons from the p-type region into the conduction band of the n-type region
   • D) Spontaneous thermal emission of conduction electrons into the surrounding ambient atmosphere
3. 3.

   Which property of gallium arsenide (GaAs) makes it far more suitable than elemental silicon for the fabrication of light-emitting diodes (LEDs)?

   • A) GaAs has a direct bandgap, enabling efficient radiative recombination without requiring momentum-conserving phonons.
   • B) GaAs has an indirect bandgap, which suppresses non-radiative thermal losses during electron transitions.
   • C) Silicon has a direct bandgap, which causes the spontaneous reabsorption of all emitted photons by valence electrons.
   • D) Silicon has an excessively wide bandgap exceeding 5 eV, preventing charge carrier injection at normal operating voltages.
4. 4.

   How does the electrical conductivity of an intrinsic semiconductor change as temperature increases, and which band-theory mechanism explains this behavior?

   • A) Conductivity decreases because increased thermal vibrations of the crystal lattice predominantly scatter existing charge carriers.
   • B) Conductivity increases because additional thermal energy promotes more electrons across the band gap from the valence band into the conduction band.
   • C) Conductivity remains constant because the magnitude of the band gap forbids any thermal promotion of electrons at non-zero temperatures.
   • D) Conductivity decreases because thermal expansion widens the band gap, preventing valence electrons from participating in conduction.
5. 5.

   Which type of extrinsic semiconductor is produced when a pure silicon crystal is doped with phosphorus atoms, and what serves as the majority charge carriers?

   • A) p-type semiconductor, with positive electron holes as the majority charge carriers
   • B) n-type semiconductor, with mobile electrons as the majority charge carriers
   • C) p-type semiconductor, with localized donor ions as the majority charge carriers
   • D) Intrinsic semiconductor, with an equal balance of mobile electrons and holes