Sheet 8 Ans. PDF - Electronics Past Paper

S ELF -T EST ◆ 21 Shell An energy band in which electrons orbit the nucleus of an atom. Silicon A semiconductive material....

S ELF -T EST ◆ 21 Shell An energy band in which electrons orbit the nucleus of an atom. Silicon A semiconductive material. Valence Related to the outer shell of an atom. KEY FORMULA 1–1 Ne ⴝ 2n2 Maximum number of electrons in any shell TRUE/FALSE QUIZ Answers can be found at www.pearsonhighered.com/floyd. 1. An atom is the smallest particle in an element. 2. An electron is a negatively charged particle. 3. An atom is made up of electrons, protons, and neutrons. 4. Electrons are part of the nucleus of an atom. 5. Valence electrons exist in the outer shell of an atom. 6. Crystals are formed by the bonding of atoms. 7. Silicon is a conductive material. 8. Silicon doped with p and n impurities has one pn junction. 9. The p and n regions are formed by a process called ionization. SELF-TEST Answers can be found at www.pearsonhighered.com/floyd. Section 1–1 1. Every known element has (a) the same type of atoms (b) the same number of atoms (c) a unique type of atom (d) several different types of atoms 2. An atom consists of (a) one nucleus and only one electron (b) one nucleus and one or more electrons (c) protons, electrons, and neutrons (d) answers (b) and (c) 3. The nucleus of an atom is made up of (a) protons and neutrons (b) electrons (c) electrons and protons (d) electrons and neutrons 4. Valence electrons are (a) in the closest orbit to the nucleus (b) in the most distant orbit from the nucleus (c) in various orbits around the nucleus (d) not associated with a particular atom 5. A positive ion is formed when (a) a valence electron breaks away from the atom (b) there are more holes than electrons in the outer orbit (c) two atoms bond together (d) an atom gains an extra valence electron Section 1–2 6. The most widely used semiconductive material in electronic devices is (a) germanium (b) carbon (c) copper (d) silicon 7. The difference between an insulator and a semiconductor is (a) a wider energy gap between the valence band and the conduction band (b) the number of free electrons (c) the atomic structure (d) answers (a), (b), and (c) 8. The energy band in which free electrons exist is the (a) first band (b) second band (c) conduction band (d) valence band 22 ◆ I NTRODUCTION TO E LECTRONICS 9. 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  P ROBLEMS ◆ 23 (c) the boundary of a p-type and an n-type material (d) the collision of a proton and a neutron 24. The depletion region is created by (a) ionization (b) diffusion (c) recombination (d) answers (a), (b), and (c) 25. The depletion region consists of (a) nothing but minority carriers (b) positive and negative ions (c) no majority carriers (d) answers (b) and (c) PROBLEMS Answers to all odd-numbered problems are at the end of the book. BASIC PROBLEMS Section 1–1 The Atom 1. If the atomic number of a neutral atom is 6, how many electrons does the atom have? How many protons? 2. What is the maximum number of electrons that can exist in the 3rd shell of an atom? Section 1–2 Materials Used in Electronics 3. For each of the energy diagrams in Figure 1–21, determine the class of material based on relative comparisons. 4. A certain atom has four valence electrons. What type of atom is it? 5. In a silicon crystal, how many covalent bonds does a single atom form? Energy Energy Energy 䊴 F I G U R E 1– 21 Conduction band Band gap Conduction band Band gap Conduction band Overlap Valence band Valence band Valence band 0 0 0 (a) (b) (c) Section 1–3 Current in Semiconductors 6. What happens when heat is added to silicon? 7. Name the two energy bands at which current is produced in silicon. Section 1–4 N-Type and P-Type Semiconductors 8. Describe the process of doping and explain how it alters the atomic structure of silicon. 9. What is antimony? What is boron? Section 1–5 The PN Junction 10. How is the electric field across the pn junction created? 11. Because of its barrier potential, can a diode be used as a voltage source? Explain.