3.2. LIGHT-EMITTING DIODES 91
Figure 3.8: Schematic of a surface-emitting LED with a double-heterostructure geometry.In analogy with the case of optical fibers (see Section 2.4.4), the3-dB modulation
bandwidth f3dBis defined as the modulation frequency at which|H(ωm)|is reduced
by 3 dB or by a factor of 2. The result is
f3dB=√
3 ( 2 πτc)−^1. (3.2.13)Typically,τcis in the range 2–5 ns for InGaAsP LEDs. The corresponding LED mod-
ulation bandwidth is in the range 50–140 MHz. Note that Eq. (3.2.13) provides the
optical bandwidth becausef3dBis defined as the frequency at which optical power is
reduced by 3 dB. The corresponding electrical bandwidth is the frequency at which
|H(ωm)|^2 is reduced by 3 dB and is given by( 2 πτc)−^1.
3.2.4 LED Structures.........................
The LED structures can be classified as surface-emitting or edge-emitting, depending
on whether the LED emits light from a surface that is parallel to the junction plane or
from the edge of the junction region. Both types can be made using either ap–nhomo-
junction or a heterostructure design in which the active region is surrounded byp- and
n-type cladding layers. The heterostructure design leads to superior performance, as it
provides a control over the emissive area and eliminates internal absorption because of
the transparent cladding layers.
Figure 3.8 shows schematically a surface-emitting LED design referred to as the
Burrus-typeLED [22]. The emissive area of the device is limited to a small region
whose lateral dimension is comparable to the fiber-core diameter. The use of a gold
stud avoids power loss from the back surface. The coupling efficiency is improved by