SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

4.9. LIGHT EMITTING DIODE (LED) 195

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

100

50

20

10

5

2

1

0.5

0.2

0.1

0.05

–OH absorption

peaks Infrared

absorption tail

from lattice

transitions

Rayleigh

scattering

FIBER ATTENUATION

(dB

km

–1

)

WAVELENGTHS,MICRONS

1.55μm loss ~ 0.2 dB/km

1.3μm loss ~ 0.5 dB/km

Figure 4.30: Optical attenuation vs. wavelength for an optical fiber. Primary loss mechanisms
are identified as absorption and scattering.

(which do not benefit from the above mentioned advantages of GaN and InGaN) are GaAs and
InP. A few semiconductors and their alloys can match these substrates. The lattice constant of an
alloy is the weighted mean of the lattice constants of the individual components, i.e., the lattice
constant of the alloy AxB 1 −xis

aall=xaA+(1−x)aB (4.9.1)

whereaAandaBare the lattice constants of A and B. Semiconductors that cannot lattice-match
with GaAs or InP have an uphill battle for technological success. The crystal grower must learn
the difficult task of growing the semiconductor on a mismatched substrate without allowing
dislocations to propagate into the active region.
Important semiconductor materials exploited in optoelectronics are the alloy GaxAl 1 −xAs,
and AlGaInP which is a quaternary material which is lattice-matched very well to GaAs sub-
strates; In 0. 53 Ga 0. 47 As and In 0. 52 Al 0. 48 As, which are lattice-matched to InP; InGaAsP, whose
composition can be tailored to match with InP and can emit at 1.55μm; and GaAsP, which has
a wide range of bandgaps available.