Callister - An introduction - 8th edition

858 • Chapter 21 / Optical Properties
photoconductivity
21.12 PHOTOCONDUCTIVITY
The conductivity of semiconducting materials depends on the number of free electrons
in the conduction band and also the number of holes in the valence band, according
to Equation 18.13. Thermal energy associated with lattice vibrations can
promote electron excitations in which free electrons and/or holes are created, as
described in Section 18.6. Additional charge carriers may be generated as a consequence
of photon-induced electron transitions in which light is absorbed; the attendant
increase in conductivity is called photoconductivity. Thus, when a specimen
of a photoconductive material is illuminated, the conductivity increases.
This phenomenon is used in photographic light meters. A photoinduced current
is measured, and its magnitude is a direct function of the intensity of the incident
light radiation, or the rate at which the photons of light strike the photoconductive
material. Of course, visible light radiation must induce electronic transitions in the
photoconductive material; cadmium sulfide is commonly used in light meters.
Sunlight may be directly converted into electrical energy in solar cells, which
also employ semiconductors. The operation of these devices is, in a sense, the reverse
of that for the light-emitting diode. A p–n junction is used in which photoexcited
electrons and holes are drawn away from the junction, in opposite directions,
and become part of an external current, as illustrated in the (a) chapter-opening
diagram for this chapter.
Concept Check 21.7
Is the semiconductor zinc selenide (ZnSe), which has a band gap of 2.58 eV, photoconductive
when exposed to visible light radiation? Why or why not?
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
21.13 LASERS
laser
All the radiative electron transitions heretofore discussed are spontaneous; that is,
an electron falls from a high energy state to a lower one without any external provocation.
These transition events occur independently of one another and at random
times, producing radiation that is incoherent; that is, the light waves are out of phase
with one another. With lasers, however, coherent light is generated by electron transitions
initiated by an external stimulus; in fact, laser is just the acronym for light
amplification by stimulated emission of radiation.
Although there are several different varieties of laser, the principles of operation
are explained using the solid-state ruby laser. Ruby is simply a single crystal
of Al 2 O 3 (sapphire) to which has been added on the order of 0.05% Cr 3 ions. As
previously explained (Section 21.9), these ions impart to ruby its characteristic red
color; more important, they provide electron states that are essential for the laser
to function. The ruby laser is in the form of a rod, the ends of which are flat, parallel,
and highly polished. Both ends are silvered such that one is totally reflecting
and the other partially transmitting.
The ruby is illuminated with light from a xenon flash lamp (Figure 21.13).
Before this exposure, virtually all the Cr 3 ions are in their ground states; that is,
electrons fill the lowest energy levels, as represented schematically in Figure 21.14.
However, photons of wavelength 0.56 m from the xenon lamp excite electrons
from the Cr 3 ions into higher energy states. These electrons can decay back into