Spontaneous emission of radiation in the visible and infrared regions of the spectrum from a forward-biased p–n junction was discussed. The normally empty conduction band of the semiconductor is populated by electrons injected into it by the forward current through the junction, and light is generated when these electrons recombine with holes in the valence band to emit a photon. This is the mechanism by which light is emitted from an LED, but stimulated emission is not encouraged, as it is in the injection laser, by the addition of an optical cavity and mirror facets to provide feedback of photons.
The LED can therefore operate at lower current densities than the injection laser, but the emitted photons have random phases and the device is an incoherent optical source. Also, the energy of the emitted photons is only roughly equal to the band gap energy of the semiconductor material, which gives a much wider spectral line width (possibly by a factor of 100) than the injection laser. The line width for an LED corresponds to a range of photon energy between 1 and 3.5KT, where K is Boltzmann’s constant and T is the absolute temperature.
This gives linewidths of 30 to 40 nm for GaAs-based devices operating at room temperature. Thus the LED supports many optical modes within its structure and is therefore often used as a multimode source, although the coupling of LEDs to single-mode fibers has been pursued with success, particularly when advanced structures are employed. Also, LEDs have several further drawbacks in comparison with injection lasers.
ü generally lower optical power coupled into a fiber (microwatts);
ü usually lower modulation bandwidth;
ü harmonic distortion.
However, although these problems may initially appear to make the LED a less attractive optical source than the injection laser, the device has a number of distinct advantages which have given it a prominent place in optical fiber communications:
ü Simpler fabrication. There are no mirror facets and in some structures no striped geometry.
ü Cost. The simpler construction of the LED leads to much reduced cost which is always likelyto be maintained.
ü Reliability. The LED does not exhibit catastrophic degradation and has proved far lesssensitive to gradual degradation than the injection laser. It is also immune to self-pulsation and modal noise problems.
ü Generally less temperature dependence. The light output against current characteristic is lessaffected by temperature than the corresponding characteristic for the injection laser. Furthermore, the LED is not a threshold device and therefore raising the temperature does not increase the threshold current above the operating point and hence halt operation.
ü Simpler drive circuitry. This is due to the generally lower drive currents and reducedtemperature dependence which makes temperature compensation circuits unnecessary.
ü Linearity. Ideally, the LED has a linear light output against current characteristic, unlike theinjection laser. This can prove advantageous where analog modulation is concerned.
The planar LED is the simplest of the structures that are available and is fabricated by either liquid- or vapor-phase epitaxial processes over the whole surface of a GaAs substrate. This involves a p-type diffusion into the n-type substrate in order to create the junction illustrated in Figure 3.1. Forward current flow through the junction gives Lambertian spontaneous emission and the device emits light from all surfaces. However, only a limited amount of light escapes the structure due to total internal reflection, and therefore the radiance is low.
The absence of optical amplification through stimulated emission in the LED tends to limit the internal quantum efficiency (ratio of photons generated to injected electrons) of the device. Reliance on spontaneous emission allows nonradiative recombination to take place within the structure due to crystalline imperfections and impurities giving, at best, an internal quantum efficiency of 50% for simple homojunction devices. However, as with injection lasers, double-heterojunction (DH) structures have been implemented which recombination lifetime measurements suggest give internal quantum efficiencies of 60 to 80%.
The power generated internally by an LED may be determined by consideration of the excess electrons and holes in the p- and n-type material respectively (i.e. the minority carriers) when it is forward biased and carrier injection takes place at the device contacts. The excess density of electrons Dn and holesDp is equal since the injected carriers are created and recombined in pairs such thatcharge neutrality is maintained within the structure.