GTBL042-19 GTBL042-Callister-v2 September 13, 2007 13:59Revised Pages776 • Chapter 19 / Optical PropertiesMATERIALS OF IMPORTANCE
Light-Emitting Diodes
I
n Section 12.15 we discussed semiconductorp-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 ap-n
junction diode, visible light (or infrared radiation)
will be emitted. This conversion of electrical energy
electro- into light energy is termedelectroluminescence,
luminescence and the device that produces it is termed alight-
emitting diode (LED).The forward-biased poten-
light-emitting
diode (LED) tial attracts electrons on then-side toward the
junction, where some of them pass into (or are
“injected” into) thep-side (Figure 19.11a). Here,
(b)BatteryPhoton
emitted+
+++
+++
+
+
–
-
- –
+ –p-side n-sideRecombination (annihilation of electron)(a)Battery+
+++
+++
+
+
–
-
- –
+ + – –p-side n-sideInjection of electron into p-sideFigure 19.11 Schematic diagram of a forward-biased
semiconductorp-njunction showing (a) the injection of
an electron from then-side into thep-side, and (b) the
emission of a photon of light as this electron
recombines with a hole.the electrons are minority charge carriers, and as
such, they “recombine” with, or are annihilated by
the holes in the region near the junction, according
to Equation 19.17, where the energy is in the form
of photons of light (Figure 19.11b). An analogous
process occurs on thep-side—i.e., holes travel to
the junction, and recombine with the majority elec-
trons on then-side.
The elemental semiconductors, silicon and ger-
manium, are not suitable for LEDs due to the de-
tailed 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.,
GaAsxP 1 −x, wherexis a small number less than
unity) are frequently used. Furthermore, the wave-
length (i.e., color) of the emitted radiation is re-
lated to the band gap of the semiconductor (which
is the normally the same for bothn- andp-sides
of the diode). For example, red, orange, and yel-
low colors are possible for the GaAs–InP system,
and blue and green LEDs have been developed
using (Ga,In)N semiconducting alloys. Thus, with
this complement of colors, full-color displays are
possible using LEDs.
Several important applications for semicon-
ductor LEDs include digital clocks and illuminated
watch displays, optical mice (computer input de-
vices), and film scanners. Electronic remote con-
trols (for televisions, DVD players, etc.) also em-
ploy LEDs that emit an infrared beam; this beam
transmits coded signals that are picked up by de-
tectors 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
(since there is no filament that can burn out). Most
new traffic control signals use LEDs instead of in-
candescent lights.(^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 12.21. In addition, Figure
12.22 shows the current-versus-voltage behavior for ap-njunction.