9.3.1 Light-Emitting Diodes (LEDs)

A light-emitting diode is a semiconductor diode that is forwardbiased so carriers recombining at a junction spontaneously emit light. That light is spontaneous emission, incoherent light like that emitted from a light bulb.

The nominal wavelength of the spontaneous emission depends on the semiconductor bandgap energy, but its range is not as strongly constrained as in a laser. Figure 9-5 shows the range of wavelengths from three typical visible LEDs.

Table 9-1 lists a sampling of LED materials and the wavelengths they produce. The bandgap of a compound semiconductor depends on the mixture of materials it contains, so the output wavelength depends on their composition. Specific wavelengths are the peaks of particular blends; a range shows wavelengths available from various compositions.

LEDs radiate spontaneous emission in all directions from their junction layer, as shown in Figure 9-6. To get the most efficient output, the junction should be close to the surface of the device. This reduces the chance that the emitted light will be absorbed by other parts of the device. A transparent lens can be fabricated on top of an LED to concentrate the emitted light, usually perpendicular to the surface. Many LEDs are packaged in ways that concentrate light in one direction.

Unlike semiconductor lasers, LEDs lack the resonant cavities needed to build up stimulated emission, so LED emission is less in-

tense than a laser beam. LEDs also emit light across a wider range of angles than lasers. Both factors make them more desirable than lasers for some applications, especially illumination, displays, and other devices intended for viewing with the human eye. Because this is a book about lasers, we will not go into much detail on LEDs, but it is helpful to understand how they work and how they are used.

Early LEDs were intended for use as indicator lights or dotmatrix displays of single-color elements. The first pocket calculators displayed numbers on arrays of red LEDs.

Now the range has broadened to include illumination and displays. Look carefully at red and green traffic signals, and you may see arrays of bright spots if they are made of LED arrays. So are red signal lights on new cars. Red, green, and blue LEDs can be combined to make white light sources for illumination; alternatively, a bright blue LED can illuminate phosphors that emit green and red light to create a white light source. Today's white LED lamps are used where weight and low power consumption are critical, such as headlamps for night-time campers and hikers. As they improve, their applications will expand into general illumination.

Organic LEDs can be made in arrays of red, green, and blue emitters to make bright color displays that consume little power. So far, they are limited to small areas, such as cell-phone displays, but larger displays have been demonstrated.