2.3 Light emitting diode  (Page 2/2)

Thus, a semiconductor with a 2 eV band-gap should give off light at about 620 nm (in the red). A 3 eV band-gapmaterial would emit at 414 nm, in the violet. The human eye, of course, is not equally responsive to all colors ( [link] ). The materials which are used for important light emitting diodes (LEDs) for each of the different spectral regions are also shown in [link] .

It is worth noting that a number of the important LEDs are based on the GaAsP system. GaAs is a direct band-gap semiconductorwith a band gap of 1.42 eV (in the infrared). GaP is an indirect band-gap material with a band gap of 2.26 eV (550 nm,or green). Both As and P are group V elements. (Hence the nomenclature of the materials as III-V (or 13-15) compoundsemiconductors.) We can replace some of the As with P in GaAs and make a mixed compound semiconductor GaAs _1-x P _x . When the mole fraction of phosphorous is less than about 0.45 the band gap is direct, and so we can "engineer" thedesired color of LED that we want by simply growing a crystal with the proper phosphorus concentration! The properties of theGaAsP system are shown in [link] . It turns out that for this system, there are actuallytwo different band gaps, as shown in [link] . One is a direct gap (no change in momentum) and the other is indirect. In GaAs, thedirect gap has lower energy than the indirect one (like in the inset) and so the transition is a radiative one. As we startadding phosphorous to the system, both the direct and indirect band gaps increase in energy. However, the direct gap energyincreases faster with phosphorous fraction than does the indirect one. At a mole fraction x of about 0.45, the gap energies cross over and the material goes from being a direct gapsemiconductor to an indirect gap semiconductor. At x = 0.35 the band gap is about 1.97 eV (630 nm), and so we would only expect to get light up to the red using the GaAsPsystem for making LED's. Fortunately, people discovered that you could add an impurity (nitrogen) to the GaAsP system, whichintroduced a new level in the system. An electron could go from the indirect conduction band (for a mixture with a mole fractiongreater than 0.45) to the nitrogen site, changing its momentum, but not its energy. It could then make a direct transition tothe valence band, and light with colors all the way to the green became possible. The use of a nitrogen recombinationcenter is depicted in the [link] .

If we want colors with wavelengths shorter than the green, we must abandon the GaAsP system and look for more suitablematerials. A compound semiconductor made from the II-VI elements Zn and Se make up one promising system, and severalresearch groups have successfully made blue and blue-green LEDs from ZnSe. SiC is another (weak) blue emitter which iscommercially available on the market. Recently, workers at a tiny, unknown chemical company stunned the "display world" byannouncing that they had successfully fabricated a blue LED using the II-V material GaN. A good blue LED was the "holygrail" of the display and CD ROM research community for a number of years. Obviously, adding blue to the already workinggreen and red LED's completes the set of 3 primary colors necessary for a full-color flat panel display. Furthermore, using a blue LED or laser in aCD ROM would more than quadruple its data capacity, as bit diameter scales as λ, andhence the area as λ ² .