"On February 23rd 1940 Russell Ohl, a researcher at Bell Labs who bore a distinct resemblance to the actor Wallace Shawn, shone a bright light onto an odd rod of silicon he was investigating. A current immediately began to flow between the electrodes stuck to the rod's ends. When Ohl put a fan in between the light and the rod the current started oscillating to the shadow-light-shadow-light rhythm of the fan's eclipsing blades. The rod's odd electrical behaviour was demonstrably down to the light.
That light could drive currents in some materials had been known since the 19th century; it was one of the things for which Einstein's paper of 1905 provided a general explanation. What distinguished Ohl's observation was that at Bell Labs he and his colleagues had the right tools, physical and conceptual, to make sense of how it was happening and how to improve its efficiency.
They discovered that the odd rod contained what came to be called a "p-n junction"--an internal electric field created by two slightly different types of silicon abutting one another. It sounds like a minor defect, but today it is more or less as fundamental to civilisation as the wheel. Applied in the field of electronics, the p-n junction changed AT&T's world from one of vacuum tubes, physical switches and operators working telephone exchanges to one embodied in slivers of silicon. Configured to make use of light, it is now helping to free the world of the need for fossil fuel.
Making such junctions is a matter of "doping" silicon by adding traces of other elements. Atoms in crystals are tied together by chemical bonds made of shared outer electrons. In a crystal of pure silicon, each atom uses its four outer electrons to make four such bonds to four neighbours. Since electrical conductors depend on free-flowing electrons, pure silicon, with all its electrons tied up, is an electrical insulator.
Dope the silicon with phosphorus and that changes: the insulator becomes a semiconductor. Phosphorus has five outer electrons, compared with silicon's four. When a phosphorous atom finds itself in a silicon lattice, four of those electrons will form bonds with its four silicon neighbours. But the fifth will be free to roam, and thus to conduct current. Doping which adds electrons in this way is known as n-type. For p-type doping you add an atom like boron, which has only three outer electrons. Now the lattice has holes in it. Those holes can also, like spare electrons, move through the lattice carrying current.
By an accident of its manufacture, Ohl's sample had layers of p- and n-type silicon right next to each other. While normally both types of doped silicon would pass some current, in conjunction, surprisingly, they could not--at least, not without light. This was because electrons in the n-type silicon had diffused into the p-type, with holes going the other way. These displaced charge carriers created an electric field, and that electric field formed a barrier no further electrons could pass.
Shine a light on such a junction, though, and the photons will knock loose fresh electrons--which, if in the junction, will flow in response to the electric field. If you put a metal electrode on the surface of the n-type silicon, another on the surface of the p-type silicon, and run a wire between them, the electrons will flow along that wire to the p-type silicon, where they will recombine with holes moving in the opposite direction: behold, a current.
Though its sensitivity to light brought the p-n junction to Ohl's attention, it was not its initial claim to fame. Bell Labs wanted to find a way of replacing the vacuum tubes and other paraphernalia on which AT&T's business depended. The p-n junction proved just the ticket. In 1947 colleagues of Ohl's came up with a device in which the insulating effect of a p-n junction could be manipulated with a second electric field, thus creating an on/off switch: the transistor. Transistors became the basis of new, cheap "solid-state" electronic circuitry. Technology which crammed many of them on to a single piece of semiconductor spawned the silicon chip.
Solar cells are made in a similar way to silicon chips but are much less complex. Their junctions just sit there, always on, turning energy from photons into electricity." [1]
1. "Gradually, then all at once... Solar power." The Economist, 9 Jan. 2021, p. 7(US).
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