|Fig. 1: Schematic of a Blue LED device geometry, which consists of several GaN-based compounds to form a pn-junction.  (Courtesy of Springer Verlag)|
Over half a century ago, the first demonstration of a light-emitting diode (LED)- based semiconductor laser was reported on gallium arsenide crystals.  Consisting of a stack of precisely doped semiconductor materials, the LED operates as a pn-junction, where a p-type material doped with impurities to create deficiencies of valence electrons (holes) is adjacent to an n-type material doped to contribute free electrons.  Electrical voltage drives electron-hole recombination that leads to the emission of photons, and the light's wavelength depends entirely on the type of semiconductor used.  This discovery led to a burgeoning LED market, which is predicted to exceed $15 billion per year in revenue over the next decade.  However, it was not until the early 1990s that researchers were able to successfully grow sufficiently clean gallium nitride (GaN) crystals to create the first blue LEDs (Fig. 1).
Red and green diodes emit longer wavelengths of light and are more readily fabricated with conventional semiconductor materials. GaN-based compounds, widely regarded as too defect-sensitive and prone to carrier trapping to operate as a pn-junction, were required to generate the short wavelength emission for blue diodes. Shuji Nakamura disproved this notion by first growing GaN crystals and then creating a InGaN/AlGaN blue LED. [5,6] His breakthrough completed the LED rainbow, allowing for true white-light-emitting devices that were an order of magnitude more efficient and longer-lasting than standard lightbulbs at the turn of the century.  Nakamura earned a share of the 2014 Nobel Prize in Physics with two other Japanese scientists for the invention of efficient blue laser diodes that have enabled bright and energy-saving white light sources. 
|Fig. 2: Comparison of common approaches to produce white light using LEDs.  (Courtesy of the U.S. Department of Energy)|
The ultimate goal of LED lighting is to emit a spectrum that most closely matches that of sunlight. Although combining red, green, and blue LEDs proves to be quite effective and efficacious, this option is also the most expensive.  A more cost-friendly option involves the use of phosphors, which are materials that absorb light of a shorter wavelength while emitting at a longer wavelength.  Blue LEDs with yellow phosphors and ultraviolet LEDs with blue and yellow phosphors are two of the most popular devices, and the latter is more similar to the Sun's spectrum. 
Unfortunately, ultraviolet LEDs with phosphor mixtures have a quantifiably lower energy efficiency.  None of these solutions is a panacea. That said, LED lighting has seen a meteoric rise in efficiency and cost reduction since its inception a couple decades ago. Reminiscent of Moore's Law for microelectronics, LED performance follows an exponential trend called Haitz's law, which predicts every decade the amount of light generated by an LED increases by a factor of twenty while the cost per lumen decreases by a factor of ten.  When comparing this relative improvement in efficacy over time compared to the historic development of white-light sources, the result is striking (Fig. 3).
|Fig. 3: Historical improvements in efficiency of LED technologies, graphed on a logarithmic scale. The linear increase in the LED curve shows the rapid efficiency improvements based on Haitz's law. After Tsao. |
Out of five billion bulb sockets in the US today, only about 2% are filled with LED-based lights.  Current LED lights require a significant up-front cost, with bulbs varying from $20-40. The purchase of a single solid-state bulb equals a several year supply of incandescents for an entire home.  Myopic consumers will overlook the 50,000 hour lifetime of high-power white LEDs and the eventual savings for not needing replacement bulbs.  It is common to see LEDs used for applications that require a substantial cost to change, such as traffic signals and automobile headlights. In terms of a widespread overhaul of the world's lighting, a shift would likely have unintended consequences. For instance, introducing the more energy-efficient LED lighting technology will likely result in a substantial increase in consumption, especially in developing countries, where the volume of demand may outpace the benefits in efficiency.  The "cleaner future" argument for switching to LEDs does not account for the entire situation. In another sense, this also applies to the misconception that LED lightbulbs are perfectly safe compared to mercury- containing compact fluorescent lighting (CFL) since LEDs contain toxic III-V semiconductor compounds and lead.  Another potential hazard with the integration of blue LEDs in lighting occurs in the attraction of insects and other pests with specific photoreceptors for blue light. One study showed that on average, a number of different white LEDs attracted almost 50 percent more flying invertebrates than sodium lamps. 
The great Yogi Berra once said, "It's tough to make predictions, especially about the future." However, the prospect that LED lighting will eventually take hold of the market is not terribly unreasonable for various reasons. First of all, Haitz's law shows stunning bounds of innovation experienced by the LED industry, which brings up comparisons to the semiconductor industry as a whole. In the past few decades, semiconductors have found their way into almost every aspect of our lives: communications, data storage, digital imaging, and optoelectronics are just a few.  Eventually, the price point of LED bulbs will reach a sufficiently competitive target that no longer spurns consumers from fronting the cost. The same process occurred for CFLs at about $14, which led to mass consumer adoption.  Efficiency mandates from the government, such as the 2007 Clean Energy Act, and subsidies to incentivize more energy-efficient lighting sources will also aid the process, much as they already have with the market share of hybrid and electric vehicles. Finally, solid-state lighting technologies will inevitably experience advancements that lead to customized and tunable settings, such as adjusting the chromaticity and the temporal/spatial location of light. 
© Nick Rolston. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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