Semiconductor electroluminescence was first reported by H.J. Round in 1907, and the first light-emitting diode (LED) was reported by O.V. Losev in 1927. Not until the birth of semiconductor physics in the 1940s and 1950s, however, was scientific development of technologies for light emission possible.
For solid-state lighting (SSL), using semiconductor electroluminescence to produce visible light for illumination, the seminal advances were
- demonstration of red light emission by N. Holonyak in 1962; and
- demonstration of a bright blue LED by S. Nakamura in 1993, along with earlier materials advances by I. Akasaki and H. Amano.
Here we briefly discuss these two advances and their subsequent evolution; more details can be found in “Light Emitting Diodes” and
Red LEDs: Ever increasing efficiencies and powers
“Solid-state lighting: ‘The case’ 10 years after and future prospects.”
Blue LEDs: Enabling white light
As mentioned above, the first seminal advance in visible light emission was in the red portion of the spectrum, and this is the LED color that dominated the early history of LEDs. The first commercial LED lamps were introduced in 1968: indicator lamps by Monsanto and electronic displays by Hewlett-Packard. The initial performance of these products was poor, around 1 mlm at 20 mA, in part because the only color available was deep red where the human eye is relatively insensitive. Since then, steady, even spectacular, progress has been made in efficiency, lumens per package, and cost per lumen.
Progress in efficiency, illustrated in the top panel of the graphic, was largely an outcome of the exploration and development of new semiconductor materials: first GaP and GaAsP, then AlGaAs, then, finally, AlInGaP. Luminous efficacies improved by more than three orders of magnitude: from about 1/50 lm/W in the 1970s in GaP and GaAsP LEDs; to 10 lm/W in 1990 from AlGaAs LEDs (for the first time exceeding that of equivalent red filtered incandescent lamps); to the current state-of-the-art of >150 lm/W 60 lm/W from AlInGaP LEDs.
The progress in lumens per package and cost per lumen, illustrated in the Haitz’ Law plots in the bottom panel of thegraphic, was enabled both by the progress in efficiency as well as by progress in high-power packaging. In 1968, red LEDs were viewable only if competing with dim indoor lights; by 1985, they were viewable in bright ambient light, even in sunlight. Nevertheless, red LEDs at that time were still limited to small-signal indicator and display applications requiring less than 100 mlm per indicator function or display pixel. Then, around 1985, red LEDs stepped beyond those small-signal applications and entered, beginning with the newly required center high-mount stop light (CHMSL) in automobiles, the medium-flux power signaling market with flux requirements of 1
-100 lm. At this point , red LEDs are well into the >100 lm high-flux domain associated with lighting-class applications.
Of course, it was not just that increasingly higher efficiency enabled these increasingly higher flux applications; the needs of these higher flux applications also drove the quest for higher efficiency. In other words, there was a co-evolution of higher efficiency and of the power-signaling markets that could make use of (and demanded) higher efficiency. Solutions based on large numbers of small-signal lamps were too expensive; markets
demanded development of higher-efficiency higher-power LEDs. Higher -efficiency higher-power LEDs, in turn, opened up additional steppingstone markets. The result is the Haitz’ Law evolution illustrated in the botom graphic. In a Moore’s-law-like fashion, flux per lamp has increased 30x per decade while cost per lumen (the price charged by LED suppliers to original equipment manufacturers, or OEMs) has been decreasing 10x per decade.
As mentioned above, the second seminal advance in visible LEDs was the blue LED, and this is the color that came to dominate the subsequent history of LEDs. The initial breakthroughs came in the late 1980s and early 1990s, with the discoveries by Isao Akasaki and Hiroshi Amano that a previously recalcitrant wide-bandgap semiconductor, GaN, could be p-type doped
and could be grown with reasonable quality on lattice-mismatched sapphire. Building on these discoveries, in 1993 Shuji Nakamura at Nichia Chemical Corporation demonstrated a bright blue LED. As illustrated in the top graphic, efficiency improvements followed quickly, to the point where state-of-the-art blue LEDs, at least at low power densities, have power-conversion efficiencies exceeding 80%.
Most importantly, because blue is at the short-wavelength (high-energy) end of the visible spectrum, it proved possible to “downconvert” blue light into green, yellow and even red light using passive phosphorescent and fluorescent materials. The visible spectrum could thus be filled out, white light could be produced, and general illumination applications became a possibility. Indeed, as illustrated in the bottom graphic, Haitz’ Law, developed originally for red LEDs, is continuing for white LEDs. There is now virtually no question that SSL will eventually displace its predecessor technologies in general illumination applications, and indeed in virtually every application in which visible light is needed.