To be useful in fiber transmission a

To be useful in fiber transmission applications an LED must have a high
radiance output. a fast emission response time, and a high quantum efliciency.
its radiance (or brightness) is a measure in watts of the optical power radiated
into a unit solid angle per unit area of the emitting surface. High radianccs are
necessary to couple sufliciently high optical power levels into a fiber. as shown
in detail in Chap. 5. The emission response time is the time delay between the
application of a current pulse and the onset of optical emission. As we discuss
in Secs. 4.2.4 and 4.3.5, this time delay is the factor limiting the bandwidth with
which the source can be modulated directly by varying the injected current. The
quantum efficiency is related to the fraction of injected electron-hole pairs that
recombine radiativcly. This is defined and described in detail in Sec. 4.2.3.

To achieve a high radiance and a high quantum elficiency, the LED
structure must provide a means of confining the charge carriers and the
stimulated optical emission to the active region of the pn junction where
radiative recombination takes place. Carrier confinement is used to achieve a
high level of radiative rccornhination in the active region of the device, which
Yields a high quantum efficiency. Optical confinement is of importance for
Preventing absorption of the emitted radiation by the material surrounding the
Pn junction.