22 ◆ GIỚI THIỆU VỀ CÁC THIẾT BỊ ĐIỆN TỬ9. In a semiconductor crystal, the atoms are held together by (a) the interaction of valence electrons (b) forces of attraction (c) covalent bonds (d) answers (a), (b), and (c) 10. The atomic number of silicon is (a) 8 (b) 2 (c) 4 (d) 14 11. The atomic number of germanium is (a) 8 (b) 2 (c) 4 (d) 32 12. The valence shell in a silicon atom has the number designation of (a) 0 (b) 1 (c) 2 (d) 3 13. Each atom in a silicon crystal has (a) four valence electrons (b) four conduction electrons (c) eight valence electrons, four of its own and four shared (d) no valence electrons because all are shared with other atoms Section 1–3 14. Electron-hole pairs are produced by (a) recombination (b) thermal energy (c) ionization (d) doping 15. Recombination is when (a) an electron falls into a hole (b) a positive and a negative ion bond together (c) a valence electron becomes a conduction electron (d) a crystal is formed 16. The current in a semiconductor is produced by (a) electrons only (b) holes only (c) negative ions (d) both electrons and holes Section 1–4 17. In an intrinsic semiconductor, (a) there are no free electrons (b) the free electrons are thermally produced (c) there are only holes (d) there are as many electrons as there are holes (e) answers (b) and (d) 18. The process of adding an impurity to an intrinsic semiconductor is called (a) doping (b) recombination (c) atomic modification (d) ionization 19. A trivalent impurity is added to silicon to create (a) germanium (b) a p-type semiconductor (c) an n-type semiconductor (d) a depletion region 20. The purpose of a pentavalent impurity is to (a) reduce the conductivity of silicon (b) increase the number of holes (c) increase the number of free electrons (d) create minority carriers 21. The majority carriers in an n-type semiconductor are (a) holes (b) valence electrons (c) conduction electrons (d) protons 22. Holes in an n-type semiconductor are (a) minority carriers that are thermally produced (b) minority carriers that are produced by doping (c) majority carriers that are thermally produced (d) majority carriers that are produced by doping Section 1–5 23. A pn junction is formed by (a) the recombination of electrons and holes (b) ionization
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