In geologic terms many regard the current epoch as the Anthropocene, based on our various far-reaching (and often obscene) effects on earth's biosphere and geology. But where are we in the sequence of cultural epochs, starting from the stone age, and continuing through the bronze and iron ages? This somewhat antiquated system of material culture-based divisions seems to have petered out with the iron age, about 500 BC. What came after? There was certainly a technological hiatus in the West (and perhaps elsewhere) around the dark ages, where iron remained the most advanced material, though one might make a case for concrete (a Roman invention, with extensive use in antiquity), glass, or porcelain as competitor, though the latter never had the broad impact of iron. The industrial age was perhaps founded on steel- the new material that brought us well into the twentieth century, until we hit the atomic age, an age that did not age well, sadly, and seems to be headed for the scap heap- one that will be radioactive for eons.
Now we are clearly indebted to a new element- silicon. That it is the magic ingredient in computers goes without saying. But now it is also providing the power for all those computers, in its incarnation as solar cells, as well as light for our lives, as efficient LEDs. It is incidentally intriguing that silicon resides just one row down, and in the same column, from the central element of life- carbon. They have the same valence properties, and each have unusual electronic properties. For silicon, its magic comes from being a semiconductor- able to be manipulated, and in switchable fashion, from conducting to insulating, and back again. A magic that is conjured by doping- the peppering-in of elements that have either too many valence electrons (phosphorous; n for negative) or too few (boron; p for positive). Too many, and there are extra electons that can conduct. Too few, and there are positive charges (holes) that can conduct similarly.
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| Charge and electrochemistry across the p-n junction. |
At the interface between n and p doped zones something amazing happens- a trapped electrical charge that forms the heart of both transisters and solar cells. The difference in composition between the two sides sets up conflicting forces of diffusion versus charge. Electrons try to diffuse over to the p doped side, but once they do, they set up an excess of electrons there that pushes them away again, by their negative charge. Holes from the p doped side likewise want to migrate over to the n doped side, but set up a similar zone of positive charge. This zone has a built-in electric field, but is also insulating, until a voltage going from p to n, which squeezes this zone to smaller and smaller size, making it so narrow that charge can flow freely- the diode effect. The reverse does not work the same way. Voltage going from n to p makes this boundary zone larger, and increases its insulating power. This, and related properties, gives rise to the incredibly wide variety of uses of silicon in electronics, so amplified by the ability to do all this chemistry on precisely designed, microscopic scales.
Solar cells also use a p-n doping regime, where the bulk of the silicon exposed to the sun is p-doped, and a small surface layer is n-doped. When a photon from the sun hits the bulk silicon, the photoelectric effect lets loose an electron, which wanders about and meets one of two fates. Either it recombines with a local atom and releases its photon energy as infrared radiation and heat. Or it finds the p-n junction zone, where it is quickly whisked off by the local electric field towards the positive pole, which is all the little wires on the surface of solar panels, taking electrons from the n-doped surface layer. The p-n interface has a natural field of about 0.6 volt, which, when ganged together and scaled up, is the foundation for all the photovoltaic installations which are taking over the electric grid, as a cheaper and cleaner source of electricity than any other. Silicon even plays a role in some battery technologies, helping make silicon-based solar power into a full grid power system.
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| Solar power is scaling to provide clean energy. |
Silicon gives us so much that is essential to, and characteristic of, the modern world. Like carbon, it is very abundant, not generally regarded as rare or precious. But that doesn't mean it lacks interest, let alone importance.
- Green hydrogen- a way to use all that excess solar.
- Generic drugs from India and China: rampant fraud.
- Meanwhile, an outstanding article describes the slow destruction of US pharmaceutical and public health capabilities.
