Callister - An introduction - 8th edition

856 • Chapter 21 / Optical Properties
electroluminescence
lightemitting
diode
(LED)
In Section 18.15 we discussed semiconductor p–n
junctions, and how they may be used as diodes
or as rectifiers of an electric current. 1 Furthermore,
in some situations, when a forward-biased
potential of relatively high magnitude is applied
across a p–n junction diode, visible light (or infrared
radiation) will be emitted. This conversion
of electrical energy into light energy is termed
electroluminescence, and the device that produces
it is termed a light-emitting diode (LED). The
forward-biased potential attracts electrons on
the n-side toward the junction, where some of
them pass into (or are “injected” into) the p-side
(Figure 21.11a). Here, the electrons are minority
charge carriers and therefore “recombine” with,
or are annihilated by, the holes in the region near
the junction, according to Equation 21.17, where
the energy is in the form of photons of light
(Figure 21.11b). An analogous process occurs on
the p-side—holes travel to the junction and
recombine with the majority electrons on the
n-side.
The elemental semiconductors, silicon and
germanium, are not suitable for LEDs due to the
detailed natures of their band gap structures.
Rather, some of the III-V semiconducting compounds
such as gallium arsenide (GaAs), indium
phosphide (InP), and alloys composed of these
materials (i.e., GaAs x P 1–x , where x is a small number
less than unity) are frequently used. Furthermore,
the wavelength (i.e., color) of the emitted
radiation is related to the band gap of the semiconductor
(which is the normally the same for
both n- and p-sides of the diode). For example,
red, orange, and yellow colors are possible for the
GaAs–InP system. Blue and green LEDs have also
been developed using (Ga,In)N semiconducting
MATERIALS OF IMPORTANCE
Light-Emitting Diodes
alloys. Thus, with this complement of colors, fullcolor
displays are possible using LEDs.
Several important applications for semiconductor
LEDs include digital clocks and illuminated
watch displays, optical mice (computer input
devices), and film scanners. Electronic remote controls
(for televisions, DVD players, etc.) also employ
LEDs that emit an infrared beam; this beam
transmits coded signals that are picked up by detectors
in the receiving devices. In addition, LEDs
are now being used for light sources. They are
more energy efficient than incandescent lights,
generate very little heat, and have much longer
lifetimes (because there is no filament that can
burn out). Most new traffic control signals use
LEDs instead of incandescent lights.
We noted in Section 18.17 that some polymeric
materials may be semiconductors (both
+ + –
+
+
Injection of electron into p-side
p-side
n-side
+
+
+
+
+
+
+
+
(a)
– –
–
–
–
–
–
–
Battery
Recombination (annihilation of electron)
p-side
+
+
+
+
+
+
+
+
+
–
– –
–
–
–
–
–
–
–
n-side
Photon
emitted
–
–
Battery
1 Schematic diagrams showing electron and hole distributions
on both sides of the junction, with no applied
electric potential, as well as for both forward
and reverse biases are presented in Figure 18.21. In
addition, Figure 18.22 shows the current-versusvoltage
behavior for a p–n junction.
(b)
Figure 21.11 Schematic diagram of a forward-biased
semiconductor p–n junction showing (a) the injection
of an electron from the n-side into the p-side, and
(b) the emission of a photon of light as this electron
recombines with a hole.