Teach Yourself Electricity and Electronics, 5th edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 19

1. When we apply forward bias to a P-N junction, that junction will conduct only if the potential difference between the P and N type materials equals or exceeds the forward breakover voltage. If the voltage is less than that, the junction won't conduct. The correct choice is (a).

2. Even though vacuum tubes have become almost entirely obsolete, they still find use in certain applications. Some audiophiles (that is, hi-fi audio enthusiasts) believe that amplifiers made with vacuum tubes produce better quality sound than similar amplifiers using semiconductor components exclusively. The correct answer is (a).

3. We will commonly find metal oxides, gallium arsenide, and silicon in semiconductor devices. However, we'll rarely come across a germanium-based component. The correct answer is (d).

4. Metal-oxide-semiconductor (MOS) integrated circuits (ICs) offer high-speed operation, so the correct answer is (d). Choice (a) won't work because MOS ICs don't weigh very much. Neither (b) nor (c) hold true; MOS devices need almost no current to operate, but they're vulnerable to destruction by static electricity.

5. In its purified form, selenium is a chemical element. The correct answer is (b).

6. When we reverse-bias a P-N junction, that junction won't conduct unless the potential difference between the P and N type materials equals or exceeds the avalanche voltage. When the reverse voltage does reach or exceed this critical value, the junction conducts. The correct choice is (c).

7. All three choices (a), (b), or (c) are generally true for holes. By elimination we must choose (d). When we read that selection closely, we can see that it's not only false, but irrelevant. Holes can't make dielectrics (or any other sort of material substance) because they're abstractions, not true physical particles.

8. A single electron carries a unit negative charge. A single hole carries a unit positive charge. Therefore, if we put an equal number of electrons and holes together, we get zero net charge. The correct choice is (c).

9. If we apply DC reverse bias to a P-N junction and the voltage isn't high enough to make the junction conduct, then a depletion region forms at the junction. The width of the depletion region varies directly according to the reverse-bias voltage, as long as avalanche breakdown doesn't occur. If we cut the reverse bias voltage in half, then the depletion region narrows. The correct choice is (d).

10. If we increase the reverse-bias voltage until a P-N junction conducts (we raise the DC voltage to the avalanche point or beyond), the depletion region vanishes altogether. The correct answer is (b).

11. In its natural state (that is, the way we find it in nature), we find silicon "bound up" as compounds in the earth's crust. In order to get it into pure (elemental) form, we must extract it from those compounds. The correct answer is (a).

12. All semiconductor materials exhibit photoconductive properties to some extent, so the correct choice is (d), "All of the above."

13. To get P type material from any semiconductor element, we must add an acceptor impurity so that the majority carriers become holes. The correct choice is (b).

14. The correct answer here is (a). Doping never yields a pure chemical element. We dope elemental semiconductors by adding impurities. That action can produce any of the results described in (b), (c), and (d).

15. When we look at the DC source polarity in Fig. 19-6 and compare it with the diode polarity, we see that the diode is forward-biased (negative cathode, positive anode). The meter indicates current flow (arrow pointing upward and to the right). Therefore, the voltage across the diode must exceed its forward breakover voltage. The correct answer is (c).

16. When we look at the DC source polarity in Fig. 19-6 and compare it with the diode polarity, we see that the diode is reverse-biased (positive cathode, negative anode). The meter indicates no current flow (arrow pointing upward and to the left), telling us that the voltage across the diode must be less than the avalanche voltage. The correct choice is (b).

17. The faster we can get the charge carriers to travel through a semiconductor medium, the better the resulting device will work at high AC frequencies. The correct choice is (d). The speed of the charge carriers (considered as a variable all by itself) bears no relation to the maximum current or voltage level at which a device can function, or on its static discharge characteristics.

18. When we forward-bias a P-N junction, we apply a relatively negative voltage to the N type material and a relatively positive voltage to the P type material. As a result, the N type material receives an excess of electrons and provides them to the P type material. The correct choice is (b).

19. Charge carriers in a semiconductor material can comprise either electrons or holes. Electrons are discrete, real material particles. Holes are not. Of the choices given here, only (a) "qualifies," so it's the correct answer.

20. In a reverse-biased Zener diode, we call the avalanche voltage the Zener voltage; they're the same thing in theory. The correct choice is therefore (d).