Doc Madhattan: Just a bit of blue

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One of the first classifications that you learn when you start to study the behavior of matter interacting with electricity is between conductors and insulators: a conductor is a material that easily allows the passage of electric charges; on the other hand, an insulator prevents it (or makes it difficult). It is possible to characterize these two kinds of materials through the physical characteristics of the atoms that compose them. Indeed, we know that an atom is characterized by having a positive nucleus with electron clouds which rotate around it: to characterize a material is precisely the behavior of the outer electrons, those of the external band. On the other hand, the energy bands of every atom are characterized by specific properties: there are the valence bands, where the electrons are used in the chemical bonds, and the conduction bands, where the electrons are free to move, the "mavericks" of the atom, used for ionic bonds. At this point I hope it is simple to characterize a conductive material such as the one whose atoms have electrons both in the valence band, both in the conduction band, while an insulating material is characterized by having full only the valence band.
Now, in band theory, the probability that an electron occupies a given band is calculated using the Fermi-Dirac distribution: this means that there is a non-zero probability that an insulator's electron in the valence band is promoted to the conduction band, but it is extremely low because of the large energy difference between the two levels. Moreover, there is an energy level said Fermi level that, while in the conductors is located within the conduction band, in the insulation is located between the two bands, the conduction and valence, allowing a valence electron to jump more easily in the conduction band.

As you see, the Fermi level crosses the band of metals, which in semimetals is a kind of double-degenerate band. In semiconductors the level is to cross the two valence bands and conduction: while in intrinsic semiconductors, as in insulation, is located right in the center, will be slightly shifted in p- and n-type.

If this would provide to an electron of the valence band an intermediate step in its ascent to the conduction band, in an insulator the distance between the Fermi level and the conduction band continues to be still too large to obtain a probability not appreciably null.
As we wrote years ago with Giusy Nigro and Alessandro Veltri (in a report for the lab course of the third year of physics course degree), the classification between conductors and insulators

(...) is as valid as that between good and evil in human society. Indeed, there are some materials, said semiconductor, in which the difference between the valence band and the conduction band is much smaller than the insulators.

This reduced difference, however, does not allow the semiconductor to be used in profitable applications: we must help, in some way, the semiconductor to allow the jump of the electrons, and thus to allow a more easily passage of the electric current. In order to do this, we must dope the semiconductor, that is, we must introduce inside the crystalline structure of the semiconductor some impurities. These impurities are essentially of two types: we can add atoms that are donor of electrons (in this case we speak about an n-type semiconductor) or atoms that are receptors of electrons (in this case we speak about p-type semiconductors). One of the most used doped semiconductors is the gallium arsenide, which will meet shortly, and which is made from gallium and arsenic.
Anyway, approaching two semiconductors of different types to each other will be realized a junction, which will create a potential difference within a system said diode, which is able to be traversed or not by the current according to the polarization of the electrical signal that passes through it. For example if we connect a diode to a common battery, we can make two separate circuits, one to direct polarization that allows the passage of current, and a reverse polarization that prevents the passage of the current.

Direct (left) and reverse (right) polarizations. Schemes designed by Alessandro Veltri

A particular application of the diodes are the LEDs, essentially made using gallium arsenide, which, such as lasers, they emit monochromatic light. The first LED to be made was the red in 1962 by Nick Holonyak, Jr.⁽¹⁾, then at General Electric Company, which was followed in 1972 by the yellow LED realized by George Craford, a Holonyak's student.
One of the first uses of LED display screens were digital watches and calculators, thus beginning a long process half a century (not yet in full) to the complete replacement of light bulbs invented (more or less!) by Edison. The last part of this process, in fact, provided for the completion of the LEDs on the primary colors: it was necessary to add the blue LED, which was first demonstrated in 1994 by Shuji Nakamura⁽²⁾ of the Nichia Corporation. The Nakamura's diode was based on InGaN, and the year after Isamu Akasaki and Hiroshi Amano⁽³⁾ completed the work by studying the structure of GaN. The work of the three Japanese researchers opens the way towards the fast development of white LEDs, and then for the realization of bulbs can emit an amount of light equal to and higher than that of incandescent bulbs with lower power consumption. And just the energy savings that emphasizes the motivation of the Nobel Prize for Physics in 2014 awarded, rightly, to Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura

for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources

(1) Holonyak N. & Bevacqua S.F. (1962). Coherent (visible) light emission from Ga(As1−xPx) junctions, Applied Physics Letters, 1 (4) 82. DOI: http://dx.doi.org/10.1063/1.1753706
(2) Nakamura S., Mukai T. & Senoh M. (1994). Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes, Applied Physics Letters, 64 (13) 1687. DOI: http://dx.doi.org/10.1063/1.111832
(3) Akasaki I., Amano H., Sota S., Sakai H., Tanaka T. & Koike M. (1995). Stimulated Emission by Current Injection from an AlGaN/GaN/GaInN Quantum Well Device, Japanese Journal of Applied Physics, 34 (11B) L1517. DOI: http://dx.doi.org/10.7567/jjap.34.l1517

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Just a bit of blue

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