The Blue LED

A small present I had been offered for my birthday a few weeks ago is a nice little gadget, a keyring pendant with a small lamp which delivers an intense blue light. Not that this little torch light is something completely indispensable, but I like it, just because...


I like it because the blue light conveys so glaringly a development that has taken place over the last 10 years or so, and that has brought a new electronic device from the alchemist-like labs of experimental semiconductor research to the mass-fabrication for give-aways: the blue light-emitting diode.

A light-emitting diode, or LED for short, is a special kind of semiconductor device. And the intense blue light of the small torch is not created by an incandescent lamp, but in such a solid-state semiconductor device.

In chunks of matter that consist of large amounts of atoms, the energy levels of the electrons are not arranged in discrete steps, as in an isolated atom, but are merged into bands. The most important bands are the conduction band, which can contain mobile electrons which can sustain an electrical current, and the valence band, which is usually completely filled with electrons and does not contribute to the electrical current. A semiconductor is a material with a gap between the valence and conduction bands and where at zero temperature, the valence band is completely filled, while the the conduction band is completely empty. This means that there are no electrons available which can carry a current, and that the material is an insulator. However, if the temperature rises, some electrons can be excited across the gap into the conduction band, and those electrons can transport a current. Thus, a semiconductor is a material whose electric resistivity drops with increasing temperature.

The electrical conductivity of a semiconductor can be increased by inserting impurities in the material: atoms that have more, or less, electrons than the atoms of the semiconducting material, and thus offer extra electrons which can populated the conduction band, or "suck" electrons out of the valence band, creating holes in the valence band. This way to insert extra electrons or of holes is called n- or p-type doping, respectively.

Now, if a n-doped and a p-doped semiconductor are brought into contact and an electrical current is driven across the junction by an externally applied voltage, the surplus electrons and the holes run into each other and can recombine, setting free an amount of energy roughly corresponding to the band gap. If the electron and the hole have the same momentum, this energy can be carried away by a photon, and the device emits light - it's a LED. The requirement of equal momentum is both crucial and restrictive and has as a consequence that only some n-p junctions can be used as LEDs. Typically, a maximum of the valence band has to occur at the same momentum as a minimum in the conduction band. Such a feature of the band structure is called a direct gap, and the width of the direct gap determines the colour of the light emitted by the LED.

The band structure of Gallium nitride (GaN) has a direct gap with a width of roughly 3.4 eV, corresponding to a wavelength of 365 nm in the near ultraviolet. (Source: S. Bloom et al, Phys. Status Solidi (b) 66 (1974) 161; via Landolt-Börnstein III/41A1b).

While semiconducting materials with direct band gaps corresponding to red light have been easy to handle and produce since quite a while, creating devices for the production of blue light turned out to be much more complicated. A material with a suitable band gap is Gallium nitride (GaN). The figure shows the band structure of GaN, i.e. the energy of electrons as a function of momentum. The different labels along the momentum axis denote special points in the Brillouin zone of GaN. There is a direct band gap at the so-called Gamma point, which corresponds to electrons (and holes) with zero momentum. Such a direct gap at the Gamma point is ideally suited for the use in LEDs, and the width of the gap 3.4 eV means that the light emitted will be ultraviolet, with a wavelength of 365 nm. So, this looks like an ideal material to build blue LEDs, and indeed, GaN is at the base of today's LEDs, such as those in my little lamp.


Shuji Nakamura at his desk
at UCSB Santa BarbaraHowever, there are some caveats: techniques to grow suitable films of GaN had to be found, as had methods to dope the material. The number of defects had to be reduced as much as possible, since defects trigger the recombination of electrons and holes without the production of light. The first LED emitting blue light with high efficiency over a longer time was finally developed in 1993 by a small team lead by Shuji Nakamura at the Japanese company Nichia Chemical Industries. They had grown a quite complicated, layered structure of GaN, AlGaN, and InGaN, and thus laid the foundation for a tremendously growing business:

Blue LEDs based on InGaAs are now very reliable and comparably cheap to produce. They are used in displays, beamers, for lighting, indicator lights, and advertisements. Used in combination with red, amber and green LEDs, they produce white light and are more energy efficient and reliable than incandescent lamps, which they may replace in the near future. Devices optimised for the stimulated emission of coherent blue light are used as blue Laser diodes in the Blue Ray optical data storage format.

The story of Nakamura, who was awarded the 2006 Millennium Technology Prize as the inventor of new source of light, is not less exciting: He managed to convince his boss at Nichia to pursuit his search for the blue LED based on Gallium nitride, and, as the Chairman of the International Selection committee for the Millenium Prize explains,

Shuji Nakamura is a splendid example of perseverance and dedicated research work, and of making a major breakthrough. He has worked with great determination for decades, and even severe setbacks have not prevented him from achieving something that other workers in the field regarded as almost impossible: using a reactor system of his own design to develop a solid material, in this case gallium nitride, into a powerful light source producing blue, green and white light, and also creating a blue laser.

That's the story of the little blue lamp.

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• The first paper about the blue LED is Shuji Nakamura, Takashi Mukai, and Masayuki Senoh, Applied Physics Letters 64 (1994) 1687; doi 10.1063/1.111832. A good general introduction is High-Luminosity Blue and Blue-Green Gallium Nitride Light-Emitting Diodes by H. Morkoç and S. N. Mohammad, Science 267 (1995) 51; doi 10.1126/science.267.5194.51 (subscription required for both papers).


• Blue Laser Diodes by Joachim Piprek is a freely available review about practical examples of GaN laser simulation, analysis, and optimisation, but gives also some general background on the GaN based blue LED technology.


• Nichia 's Shuji Nakamura: Dream of the Blue Laser Diode is an interesting interview with Nakamura.

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Update: via Cocktail Party Physics, here is a report covering a talk by Nakamura on the Current Status of Solid State Lighting at the American Institute of Physics Industrial Physics Forum on "The Energy Challenge" last Monday, October 15, in Seattle.

Solid-State Lighting, the use of LEDs for lighting, has huge prospects because it is more energy-efficient than using usual incandescent or fluorescent lamps, and because LEDs are becoming ever cheaper and more durable. More information can be found, e.g., in Physics Today, December 2001: The Promise and Challenge of Solid-State Lighting (doi 10.1063/1.1445547), and at this page on Solid-State Lighting of the U.S. Department of Energy - Energy Efficiency and Renewable Energy - check out the PDF files.


TAGS: physics, blue LED