Optoelectronic Semiconductor Devices Principals and Characteristics

4.2.5 
CURRENT-VOLTAGE CHARACTERISTICS
 
The current-voltage characteristics of LEDs are basically given by formula (46). In the operating bias 
range of LEDs, the diffusion current is dominant and the radiative current is given by 
0
exp
b
s F
F
act
s
B
V
R I
I
S J
k T


−
=




(46) 
where R
s
- the series resistance. 
As shown in Figure 
28.
, the current increases rapidly when the applied voltage exceeds the band-gap of 
the active layer. (0.8 V in the Fig. 
28.
). 

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5 SUPERLUMINESCENT DIODES 
The structure of a SLD is very similar to that of an ILD of edge-emitting LED. All lasers have tree basic 
components: 
• an amplifying medium; 
• an optical feedback mechanism; 
• a means to receive power from external sources. 
An SLD does not have an optical feed back or cavity, which is the main difference between SLD and 
ILD. 
The front surface on an SLD is coated with an antireflection material, which may consist of several 
dielectric layers. In some SLDs, the back surface is also coated. (Figure 
32.
). 
Figure 32.: Geometry of a superluminescent diode. 
[1]
Because of the antireflection coating on the back facet and/or the lossy segment near the rear end of an 
SLD, optical feedback and stimulated emission are greatly suppressed. 
The radiation emitted by an SLD is amplified spontaneous emission. 
Since the optical amplification occurs only in a single pass in the forward direction, the realizable gain is 
limited. In general, the current density required for SLD operation is much higher than that required for 
LEDs or ILDs. (Figure 
33.
). 

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