except for those due to lasing.
In the bias range below
that in which lasing occurs, the diffusion current is approximately given by
formula
0
exp
b
s F
F
act
s
B
V
R I
I
S J
q
k T
−
=
(25)
As
shown on the Figure
22.
, the injection carrier density in the active layer increases rapidly.
Figure 22.: Injected carrier density as a function of injected (forward) current.
[4]
The differential resistance is given by:
1
b
B
s
F
F
dV
k T
R
dI
q I
=
+
(26)
In the LED-mode operation before lasing, the differential resistance decreases
rapidly with injected
current and approaches the series resistance,
R
s
. At the lasing threshold current
the differential resistance
corresponding to the slope of current-voltage characteristics becomes constant as shown in Figure
22.
Most of the injected carrier is converted to lasing output power. As a result
of such rapid carrier
recombination, the carrier density on the active layer becomes constant (Figure
22.
).
The junction voltage is given by
(27)
j
b
s
V
V
R I
=
−
F
by
taking
0
j
F
dV
dI
≈ , we get:
b
s
F
dV
R
dI
=
(28)
We can see that the differential resistance changes rapidly from the value given by formula (26) to that
given by formula (28).
When
the bias is low, laser diodes operate in a manner similar to LEDs.
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