Page 454 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 35] THE SOLID STATE 439

35.10. A photon is given off by a light-emitting diode when
(a) a hole is created
(b) a hole is filled
(c) two holes collide
(d) two electrons collide

Supplementary Problems

35.1. You are given two solids of almost identical appearance, one of which is held together by ionic bonds and the other
by van der Waals bonds. How could you tell them apart?

35.2. Why can metals be deformed with relative ease whereas covalent and ionic solids are quite brittle?

35.3. Van der Waals forces hold inert-gas atoms together to form solids, but they cannot hold such atoms together to form
molecules in the gaseous state. Why not?

35.4. How good a conductor is (a) a crystal whose upper energy band is partly filled with electrons? (b) A crystal that has
a wide forbidden band between a filled lower band and an empty upper band?

35.5. Does the “gas” of freely moving electrons in a metal include all the electrons of the metal atoms?
35.6. The forbidden band in silicon is 1.1 eV wide, and in diamond it is 6 eV wide. What bearing do these figures have on
the transparency to visible light of silicon and diamond?
35.7. Semiconductors are usually opaque to visible light but are transparent to infrared light, whose frequencies are lower
than those in visible light. What is responsible for the difference in transparency?

35.8. An indium atom has three electrons in its outer shell, and a germanium atom has four. Does the addition of a small
amount of indium to germanium produce an n-ora p-type semiconductor?

35.9. At a p-n junction, (a) which direction of current represents forward bias and which reverse bias? (b) In which
direction does current flow more rapidly?

35.10. Is energy absorbed or given off when an electron and a hole recombine? If energy is absorbed, where does it come
from? If energy is given off, where does it go?

Answers to Multiple-Choice Questions

35.1. (a) 35.6. (b), (c)

35.2. (d) 35.7. (b)

35.3. (b) 35.8. (a)
35.4. (a) 35.9. (c)

35.5. (c) 35.10. (b)