Page 455 - Schaum's Outline of Theory and Problems of Applied Physics

440 THE SOLID STATE [CHAP. 35

Answers to Supplementary Problems

35.1. The van der Waals solid will be softer and will melt at a much lower temperature.

35.2. Atoms in a metal can be readily rearranged in position because the bonding occurs by means of a sea of freely
moving electrons. In a covalent crystal, the bonds are localized between adjacent atoms and must be ruptured to
deform the crystal. In an ionic crystal, the bonding process requires a configuration of alternate positive and negative
ions whose relative positions cannot be altered without breaking the crystal apart.

35.3. Such forces are too weak to hold inert-gas molecules together against the forces exerted during collisions in the
gaseous state.
35.4. (a) Excellent (b) Very poor

35.5. Only the outermost electrons in each metal atom become part of the electron “gas.”

35.6. Silicon is opaque because its electrons readily absorb photons of visible light and enter the upper energy band.
Diamond is transparent because photons of visible light do not have enough energy for electrons to absorb them and
enter the upper band.
35.7. The forbidden bands in semiconductors are comparable in width to the photon energies in visible light, hence
semiconductors absorb visible light and are therefore opaque to it. The lower frequencies in infrared light mean that
its photon energies are too low to be absorbed, hence semiconductors are transparent to such light.

35.8. p-type
35.9. (a) p → n, forward bias; n → p, reverse bias (b) Forward bias

35.10. Energy is given off and is either absorbed by the crystal as heat or is emitted as a photon of light.