As discussed in Chapter 10, a direct-gap semiconductor is a far more efficient emitter and detector of optical radiation since radiative transitions in these materials proceed to first order. In indirect-gap semiconductors, such as silicon and germanium, radiative transitions cannot proceed to first order but require a second process, phonon absorption or emission, etc., to occur. One of the most important characteristics of the compound semiconductors is that many of them are direct-gap semiconductors. For this reason, many of the compound semiconductors are used for light emitters such as light-emitting diodes (LEDs) and semiconductor diode lasers. In this chapter, we discuss optoelectronic semiconductor devices that emit photons.

Light-Emitting Diodes

We begin our discussion of optoelectronic emitters. Perhaps the simplest semi-conductor light emitter is the LED. LEDs have become pervasive because of their low cost, high efficiency, wide spectral capabilities, relatively simple drive circuitry, high reliability, and very long lifetime. LEDs are most familiar as displays and indicator lamps. However, the relatively low cost, high efficiency, long lifetime, and reliability make them attractive candidates to replace incandescent bulbs in many applications, that is, particularly in those applications in which it is difficult to replace the lamp (like automobile dashboards). Future lighting systems in automobiles, traffic lights, outdoor lighting fixtures, etc., may also utilize LEDs for these reasons.

As discussed in Chapter 10, there are two basic types of radiative transitions, spontaneous and stimulated. Stimulated emission requires the presence of an electromagnetic field to induce a transition.