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Optical Sources, Detectors, and Systems by Robert H. Kingston

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60
Chapter 3 The Semiconductor Laser
in typical laser structures obtained from Eq. (2.10) are from 20 to 80
cm~^
resulting in required electron densities of the order of lO^^cnr^.
3.3 Laser Structures and Their Parameters
Historically, the first semiconductor laser was a homo junction, a
simple configuration of
p-
and n-type material as shown in Figure 3.8.
^V
p-GaAs
n-GaAs
Figure 3.8 Homojunction laser showing typical dimensions.
A Fabry-Perot cavity is formed by the cleaved endfaces, which have a
power reflectance of approximately
307o
due to the high index of refrac-
tion of the gallium arsenide, n
=
3.5.
Gain occurs in an active region in
a thin plane at the junction. Laser oscillation will produce an output
beam at the cleaved faces because of the higher overall gain in the long
dimension. Figure 3.9 shows the energy band behavior for the diode
before and after application of forward bias. The active region at the
center of the junction has dimensions controlled by the carrier diffusion
length and is typically of the order of
3
i^m.
From Eq. (3.12), the required electron density for threshold or laser
oscillation is about
2
x
10^^
cm'^,
using a typical required threshold gain
of approximately 20
cmr'^
as detennined from Eq. (2.11). This leads to a
threshold
current
density of
3.3 Laser Structures and Their Parameters
61
U.
u =u =u
u.
F....FQ...
JPyl
./.+.+..-!-.+.
.+ .4-..+
.+.
.4:.
^
active region
FV
(b)
Figure 3.9 The homojunction laser (a) unbiassed and (b) forward-biased.
/ =
_ mnj^ _ (1,6x10-^^X2x10^^X3x10-^)
(3x10-^)
32x10^ AI
cm^
(3.1)
where
L
is the diffusion length. This is an extremely high current den-
sity and for the structure of Figure 3.8, would result in a threshold cur-
rent of 16 A! Consequently, early semiconductor lasers were only
operated in the pulsed mode and at liquid nitrogen temperatures (77 K).
The low-temperature bath significantly reduces the required electron
density and also carries away the excess heat.
The development of the
heterojunction
allowed lasers to be operated
continuously at room temperature. The heterojunction consists of dis-
similar semiconductor materials with slightly different energy gaps. In a
structure such as that shown in Figure 3.10, the carriers are confined to a
much smaller region, typically
0.1
ixm
in length in contrast to the diffus-
62
Chapter 3 The Semiconductor Laser
p-AlGaAs
i-GaAs
n-AlGaAs
hv
-fi+^
U
U.
+ + + + + + +
-I--I-
+
Figure 3.10 Heterojunction composition and energy band structure.
ion-hmited length of the homojunction, 3
/Lttn.
Electrons moving to the
right and holes moving to the left are trapped or confined to the narrow
GaAs region. For a
0.1-fxm
active region thickness, the threshold
current density becomes
10^
A/cm^ and the threshold current, 500 mA
This lower current allows operation at room temperature but with an
applied voltage of about 1.5 V, the input power of
0.75
Wis still rather
high for continuous operation. This problem can be solved by the use of
a "stripe" or "buried" junction configuration as shown in Figure 3.11.
The cross-hatched region is the active GaAs while the remainder of the
structure is again a ternary such as AlGaAs vdth a larger gap. In (a), the
metallic contact to the p-region confines the current to the center of the
junction, while in (b) the active area is confined by the insulating
material on either side. Using typical dimensions of 3
/urn
for the width
of the active region, one obtains a threshold current of only 16 ma
because of the reduced area in the direction of the current flow. Since the
AlGaAs has a lower index of refraction than GaAs, there is also a
confinement of the optical wave to the active region, especially in the
buried structure of Figure 3.11(b), where there is a reduced index to the

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