In Our Time - Conductors and Semiconductors

Episode Date: February 23, 2012

Melvyn Bragg and his guests discuss the physics of electrical conduction. Although electricity has been known for several hundred years, it was only in the early twentieth century that physicists first satisfactorily explained the phenomenon. Electric current is the passage of charged particles through a medium - but a material will only conduct electricity if its atomic structure enables it to do so. In investigating electrical conduction scientists discovered two new classes of material. Semiconductors, first exploited commercially in the 1950s, have given us the transistor, the solar cell and the silicon chip, and have revolutionised telecommunications. And superconductors, remarkable materials first observed in 1911, are used in medical imaging and at the Large Hadron Collider in Geneva. With:Frank CloseProfessor of Physics at the University of OxfordJenny NelsonProfessor of Physics at Imperial College LondonLesley CohenProfessor of Solid State Physics at Imperial College LondonProducer: Thomas Morris.

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get your podcasts. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio four. I hope you enjoy the program. Hello, until the end of the 19th century, the phenomenon of electricity was very poorly understood. But in a few generations, the world was transformed,

first by electric power and then by the electronic revolution, which resulted in invention of computer technology. The transformation was brought about by a new understanding of not just of electricity but of the materials through which it can travel. All substances on the planet can be divided into categories according to their ability to conduct electricity. Those that can, like metals, are called conductors. Those that don't are known as insulators.

But there are also semiconductors whose discovery has made possible the invention of the transistor and the solar cell. And finally, there are superconductors whose unique properties have a range of useful applications from medical imaging to particle accelerators. With me to discuss electrical conduction, semiconductors and superconductors of Frank Close, professor of physics at the University of Oxford,

Jenny Nelson, Professor of Physics at Imperial College London, and Leslie Cohen, Professor of Solid State Physics, also of Imperial College London. Frank Close, when did scientists begin to tackle and understand the phenomenon of electricity? Well, understanding, of course, is relatively recent, but the idea of the phenomenon has been around for two and a half thousand years or more. I mean, the simplest way probably of making electricity right here and now is brush your hair very violently.

And if you do that in a darkened room, you might even make sparks fly. And that sort of phenomenon was known to the ancient Greeks. They knew that if you rubbed a form of resin called amber, it would have these mysterious properties of electrical attraction. And amber, the word in Greek is electron. So the modern word electron and electronics reflects back the original. Greek's insight that Amber was a mysterious substance. We can't get away from the Greeks, can we really? Not really. Not that we want to.

Now we'll jump to the USA around the 18th century, Benjamin Franklin, who was obviously a polymath that, as well as helping draft the US Constitution, he was doing experiments on particular trying to understand the nature of electricity by flying kites in thunderstorms. Don't do this at home. And in the course of that, he had one of the first-rate insights that there are two varieties. is he called them positive and negative, that some mysterious stuff could be transferred from one body to another.

If you had it or deficient in it, plus and minus. And from that, we move really into the 19th century. They knew what electricity did, but they hadn't got a clue what it actually was or is. They knew they could flow through wires. And so the idea was maybe if we could look inside the wire, could we see what this fluid is? but how do you look inside a wire?

Well, the clever thing was that electricity can also flow through gases. By making the gas thinner and thinner, they hope to be able to see the current flowing. And very interesting things happened there that weird lights started being emanating out of the tubes, like modern neon lights today. And you can imagine these Victorians who love these sort of siennes shows. In a darkened room,

William Crooks, who was one of the leading experimenters, convinced himself he was creating ectoplasm, and he became a spiritualist, but he wasn't. And eventually these experiments led to the real key, which was when you removed all of the gas, the current could still flow and hit the screen at the front like the old-fashioned television set. And that really came to the discovery of the carriers.

Can you tell us what J.J. Thompson brought to the table, briskly, what period we're talking about and what he brought forward? J.J. Thompson, 1897, was the man who discovered the electron, the carrier, of electrical current. The basic Was he at Cambridge? He was at Cambridge then and he was firing, we now know electric current through a vacuum to hit

a screen at the front of the tube he could apply electric fields and magnetic fields and by adjusting them cleverly was able to make the beam spot move around and from this he worked out the properties of whatever it was that was carrying the current. The electron

he discovered was we now know a particle very, very light, about 2,000 times lighter than hydrogen, the lightest atom of all. And he then had the insight that electrons must exist inside the atoms of all atomic elements. So he was, in fact, the first person to discover the stuff inside atoms. So in that sense, in the idea of modern knowledge we get going, Jenny Nelson, can you develop, can you tell us a bit more about electrons? They're going to be central to this programme and let's know where we are with the little beggars. Yes, so from the early part of the first 10 years into the 20th century,

it was known that the atom consists of a nucleus, which is very small and positively charged, and that's surrounded by electrons. And these electrons occupy different states. And these states are, they have different energy, like rungs on a ladder, but not necessarily equally spaced. And that idea that electrons must. occupy states of particular energy. That was one of the main findings of quantum physics in the 1920s.

So you have this atom, and it has the states it can be occupied by electrons, and you have this family of electrons that have to be arranged in the states. Adam is, can you, Frank gave us some idea of the size. What size are we talking about with atoms? Well, an atom would be about half and about, well, it depends on which, on which atom you're dealing with, which element is. the periodic table. And smallest atom is about, well, about one billionth of a meter in size or less than that.

So one, you know. And you have the ladder image, but you could also say that the electrons are orbiting the central nucleus of the atom, a bit like the solar system orbiting the solar. Is it anything like that just as an image for people? That's right. I mean, we really need to have both concepts. The idea that they have different energies, the states of different energies. So you've got the atoms.

I interrupted you. the positive nucleus and the negative. And when they're in the lowest energy state, then they're found close to the nucleus because they're very strongly held. But when they're in the higher energy states, then they're less strongly held by the nucleus and so they can be found further away.

And so you could think of them as being in more distant planetary orbits. So the atom with the electrons, some electrons are close to the nucleus. Some electrons are further away. And the further away, the weaker their relationship with the nucleus. The nucleus is positive, the electrons are negative. Those are loosely around the edges. Those are those that are going to take us into the next stage.

That's how you discovered, people like you, how the whole caboose worked. Can you tell us why they are so important, the outermost electrons? I'm going to keep using the solar system circling. Why the outermost ones are so important? Well, the outermost ones are the ones that are easiest to remove. What do you mean by remove? Well, if you bring together two atoms together, supposing you have, we've mentioned that there are different states the electrons can occupy, and these states like to be filled. So if you have, if you bring together an atom that has a particular number of electrons where there's one electron on its own together with another atom, where there's one electron too few, then the electron can be transferred.

from the first it could be sodium, for example, to the other. It's just a force of attraction that transfers it. Well, when the... There must be some sort of attraction, so what is it? I mean, the whole system, the energy of the whole system, the two atoms together and all their energies will be lower if they come together and that electron is transferred. So it becomes more stable.

So we can think of it as a bonding force, if you like. And a sort of self-adjusting force for the atom to keep it, keep its own stability and own sense of itself. It can offload some electrons. Yeah. So the idea when you bring two electrons, two atoms together, they can share their outermost electrons.

And what happens is that each atom has got its set of levels or set of orbitals, if you like. When they come together, you get a new set. And that becomes a set for the molecule. And electrons are shared between the atoms. atoms. So if you take that further and you bring together a lot of atoms into a solid, if they're all identical, they've all got an identical set of orbitals, and those orbitals will come

together and mix, and they will form a set of bands where you have orbitals overlapping, and then there are gaps between the bands where there are none, where you can't have an electron, and then there will be more in another band. And the thing that is, you can't have an electron. And the thing that's important for conduction in the solid is the band which contains the highest occupied band and the electrons in there, they're responsible for

the electrical properties of the solid. They've got the electrical properties and when they move through a material they give an electrical charge. They give an electrical current. Electrical current, that's right, sorry, I got the word wrong. It won't be the first time in this programme. Still, we know where we are now.

We're very good, Leslie Cohn. So can we talk now about conductors and then we'll talk about how they differ from insulators but we've been taken a long way down the line by Jenny can you take us on as it were in my way of going to the next stage I can try

I think I actually would probably like to come back to some of those ideas that Jenny that Jenny's just described it's actually it may be easier to first talk about insulators actually because what Jenny's just

described is the picture of inner solid bands in which electrons are allowed to live, as it were. And a lot of physics can be understood in terms of thermodynamics or, you know, the system is in a lower energy state if it takes up a certain form. And solids would like. to have these energy bands filled up with electrons. And in materials, elemental solids that are insulating, all the electrons are filled up within one of these bands. So instead of individual atoms with orbits, we now have solids where bands have been formed across the material

because a solid is a collective property. And in an insulator, all the electrons fill up a band. And then there's a really huge gap until you get to the next band. So those electrons are sort of stuck where they are. They can't move because they would require a lot of energy before they could jump up the ladder that Jenny described in energy to the next band. So that's why some elemental materials are insulators. So ceramic is an example,

and glass and glass and diamond. So let's go on to conductors then. So some elemental solids find that it is easier to when they come together the electrons do not fully occupy the band.

In fact, they only partially occupy a particular band and that means they've got lots of room to move around in terms of energy levels within their allowed band and those materials we call metals primarily

and when the electrons when the atoms come together to form a metal the metal bond is such that the electrons in the band are basically given up to the whole solid

so that all the whole material shares, it's a collective sort of along the Russian model, a collective concept where they all share all the electrons, the electrons can move freely through the metal.

So we have how does that relate to the notion of the electric current? So an electric current and people a familiar concept about electric current is that charge flows through a wire

and indeed it's similar on the most everyday level to water flowing through a pipe you have to encourage water to flow through a pipe and you may use a pump to do that to pump water through a pipe and a smaller pipe you will have to force the water through more. And that's similar to electric current.

When you apply a battery to a loop of wire that is metallic, you basically, that battery sets up a force, an electric force across the wire, and electric charge will flow along that wire. All metals are conductors. Yes, although not all. conductors are metals, but yes, all metals are conductors.

So you set it up, you can flow it, and the water image down a pipe is a good one? Yes, because a narrow pipe, the water suffers frictional forces, and it's, you know, the water will sense resistance. In an electric,

in the electric version, electrons, these three electrons that we've just described, as they travel down the wire, will suffer. for similarly electric,

they will suffer what we call resistance. Now, the pipe model is a good one on the everyday level, but microscopically what is actually going on

is absolutely extraordinary just in that very simple situation that most people are familiar with because to really understand what's happening, even just in a simple wire, metal, one needs quantum mechanics

all sorts of very exotic concepts which developed in our times as it happens in the last hundred years the electrons will suffer collision with themselves with the positive ions of the nucleus with any disorder that's in the wire and of course impurities

right Frank, do you think it's clear enough so far? I'm learning a lot. Well, I mean, I'm asking a serious question because I think people will be riveted by this and not many people know much about advanced physics, and I'm one of them who doesn't know. But a little, well, we'll call that nothing about advanced physics.

But have we dealt with what a conductor is then? Well, I think the idea of a conductor we've probably got that there are electric charges, electrons, and if you give them an electric force, they can move around, as long as they're not trapped somewhere. Some materials like to trap the electrons, so the current can't flow, they're insulators. Other materials, the electrons are free to move around,

and if you give them an electric kick, they will flow. I think the analogy with water flowing is a very good one. I remember, in fact, when I first met electricity in school, that was the way that it was presented by the physics teacher. And it's a mental picture that I've always had. You can draw the analogy of gravity, the pipe, the water flowing downhill under the force of gravity, the electric current flowing downhill under the effect of the electric field.

So the analogies are very good. So we've got the conductor, now we come to a semiconductor, but before we go there, can you tell us at what stage people like yourselves were discovering this, when this came on the sort of intellectual map and it began to transfer into uses, and more people got to know about it? well the uses and things that's the experts in either side of me but I think to me

the way that I sort of tried to understand these things when I first met them was why do some substances act as insulators and other ones not and what are semiconductors and why do some substances turn out to be semiconductors and that I remember went back even to chemistry that the periodic table that was mentioned

that as you go through the periodic table occasionally you come across some chemicals which are inert like neon and then eight later in the table you come to argon which is also inert they're inert because the electrons like to be trapped

in there they don't want to do chemistry chemistry is electrons swapping around next to neon in the periodic table is sodium it's got one more electron and next to argon is chlorine which has got one fewer electron and what the energy likes to do is to shift that electron away so that sodium

becomes positively charged and chlorine negatively charged. And then as Jenny said earlier, the attraction of those opposite charges, the sodium positive iron and the chlorine negative iron, gives you a nice attraction. You mentioned, Melvin, you know, the use of these things, I just wanted to say that the comprehension, and indeed when we come on to semiconductors,

that couldn't have possibly been established until this band picture that Jenny mentioned was established. The use of conductors, because, you know, humankind uses things, even when we don't understand them, was done in the 1820s, 30s, 40s, by Ampare and Faraday and so on. That, you know, that was all much earlier, and they didn't understand conduction. The most critical thing to appreciate is insulators, metals, and semiconductors, and this band picture. Let's go back to Jenny then. Can we talk about the semiconductor and how?

that can be persuaded to conduct electricity. Yes, okay, well let's first define, if you like, what a semiconductor is. So we've talked both, I think we've all spoken about the idea of electrons occupying different levels or different bands. And in this picture of the band picture, if you have a semiconductor, you have a situation where there is a band that is completely full of electrons. and then there's a gap in energy and then the next band is some distance in energy away and it's empty. So if you have a semiconductor

and it's completely pure and it's dark and it's cold, it would be an insulator. It won't conduct electricity. And we could think of this maybe in analogy to something like a two-tier bridge where you've got two carriageways. And on the lower carriageway

there's a traffic jam. got lots of cars, but they're stuck in a traffic jam. They can't move. And on the upper level, you've got no vehicles at all. So nothing's happening, nothing's moving. And if you want to make some traffic move, then either you've got to put some cars on the top level, or you've got to make some space on the lower level, and that would allow traffic to flow. So in the case of the semiconductor, what we need to make it conduct is somehow to introduce electrons into this empty conduction band that's available or take some electrons out of

this lower band, the valence band, which is completely full. And there's different ways of doing that. So one way of doing it is to give it some energy. So you could do that. Can you give us an example of a semiconductor? Well, the most well-known example, of course, is silicon. And silicon and the first one to be used commercially actually was germanium.

And both of those are elements in group four of the periodic table. Sorry, back to the bridges. Back to the bridges. Well, actually, back to... So we want to make it... We want to move our electrons upstairs. So you can do that if you give it some energy.

You could heat it up, or you could shine light on it. We might come back to that. But there's another way of doing that, and that is by something called doping. And here, you introduce some foreign atoms, some different atoms, into the semiconductor. So supposing we've got silicon, Each silicon has got four outermost electrons

and they're all busy making bonds with other silicon atoms so none of them are available to conduct electricity and then if you replace one of your silicon atoms by an atom of phosphorus phosphorus has got five valence electrons so there's one left over and it's not needed it's not tightly held by the atoms so that means it's available to conduct and it actually sits in the conduction band

So if you control, and you can do this very closely in a pure semiconductor, the amount of these phosphorus or other impurities that you put in, then you can control the conductivity of your semiconductor. And moreover, you can also make it conduct if you put in different impurities which have got two few electrons, and then they make spaces that also allow. And so you can make, so doping allows your semiconductor, to conduct. But the really important thing is actually what happens when you bring two pieces of

semiconductor that are differently doped together, because that allows you to kind of create a structure which allows current to pass in one direction and not in the other direction. That's what we call a diode. And the importance of that is it allows you to control an electric current, to turn it if you like on and off, and that allows you to process information. Frank, can we talk about semiconductors that are affected by light? Can you explain why? Well, light carries energy, and light can interact with electric charge, like electrons.

And so if a photon, that's a particle of light, hits an electron, you can think of it like two billi-balls colliding. The photon can kick the electron out of the atom if there's enough energy in the photon to do that. So by light, you mean sunlight, electric light, torchlight, any other. The rainbow of light. Sorry. I might just bring in, if I may, Frank.

What's actually a very useful idea here is the experiment, the kind of celebrated experiment which led Einstein to his Nobel Prize winning theory, which was about light being made up of packets of energy called photons. And in that experiment, you saw that light of ultraviolet light was able to knock electrons off a metal plate while light of longer wavelength wasn't

and that's quite similar to what happens in a semiconductor. Yes, I mean the basic idea is exactly as Jenny says that the surprise was that red light, however intense it was, didn't seem to do anything, whereas violet lights very faint would work. And that was the apparent. and then they realised it was the colour or the wavelength or the energy that mattered, and that was what Einstein's great insight was. Relativity, yes, but the Nobel Prize was for this quite different thing. And by measuring the energies in the light that are needed to turn the electric effect on,

you can learn about the energies that the electrons are trapped with inside the materials. And so you can learn a lot about semiconductors from this sort of phenomenon. So we've got conductors, semiconductors, semiconductors, doping. and we've talked a little bit about light. Leslie Cohen, we come now to superconductors. What are they, when were they discovered? Well, really, this is a sort of chalk and cheese conversation because superconductors are metals,

which when cooled, basically enter an entirely new state of matter. People are familiar, for example, with water. and ice as two states of the same matter. And superconductivity is a new form of matter and it has unique properties. It only exists at low temperatures. What do you mean by low temperatures? Very low.

Well, it depends on your superconductor. Originally around the temperature that helium liquefies, which in the temperature scale used by low, low temperature businesses, which is Kelvin, is about 4 Kelvin, which is about minus 269. 69, thank you, degrees Celsius,

minus 263. Very cold. So a superconductor can be defined, and indeed over my lifetime, there have been a few that have been discovered. and lots of people who've claimed that they've found a superconductor and in order to claim you've really found a superconductor you have to demonstrate these two unique properties,

one of them being zero resistance to a direct current, which is extraordinary. So when you found a superconductor, what have you found? You ring up somebody like you three and you say, I found a superconductor and they say, what is it? And what do you say? What you say is, and people have. who, you know, they normally publish it. They keep it very quiet and then they publish

They don't tend to ring anybody. I apologize to introduce the vernacular to this conversation, but I want to move it. What is a superconductor? I found this superconductor over 200 degrees minus a susceptible. It carries. So it was named Superconductor

because it has the property unlike the scenario I described before, when you pass an electric current through it, the electrons are in a new state and they do not suffer any resistance. So it's a zero-resistance conductor, super, in other words. But what is it? Right.

First of all, that's only one of its two properties. The other, and I'd just like to say what it is, is that it does not allow any magnetic field in its interior and I can later perhaps tell why that's important what it is is as I said a new state of matter where electrons do a new thing basically they basically get together and couple up in pairs named Cooper pairs after Cooper, who first discovered them.

And they couple up in pairs, and all of them at the lowest temperatures will do that, and they move, as I said before, about thermodynamics, into a new lower energy state. So that's really what the superconductor is. Okay. An analogy to this that is often given is to

we drew the analogy earlier of gravity and electric forces and the analogy then that electrons are like people who are dancing so conventional conductors are like people who are jiving but on a dance floor that's sloping so they're gradually drifting across the floor

but they're jiving at random and keep bumping into each other they lose a lot of energy get very hot and that is what a conventional conductor is a lot of resistance a lot of heat dissipated the superconductors is again people dancing, but it's a very coordinated dance troupe,

like one of those old Hollywood movies where it's very strongly choreographed, so that you have a partner, but you're not dancing cheek to cheek. Your partner is somewhere across the dance floor, and that applies to everybody else in there, and the only way that can work is if the choreography is done very delicately, you and your partner have to act together.

You are, in the jargon now, the cooper pairs, two electrons acting as a unit. and that turns out to be very powerful in the way that materials behave, and I'm now beyond my paygrace. You're beyond your paygris. Thank you. Right, Jenny Nelson, right. So semiconductors are the basis of a technological revolution in the 1950s. Can you tell us why they've, can we talk about applications now?

Can you tell us why they've proved so useful in the field of electronics? Okay, well, this sort of semiconductor revolution also, started in 1947 when the semiconductor transistor was invented discovered in this was by William Shockley and his

co-workers in Bell Labs in the United States. So what a transistor is it's a device that basically involves two diodes back to back and it's able to amplify electric current and there was a lot of effort at trying to do this

trying to achieve this during the war in order to process radar signals. But the discovery actually came after the war. And so the first application, in fact, was in broadcasting because you could use semiconductor diodes and transistors to pick up and amplify radio signals. And the point was that these devices were much smaller and faster and cheaper to make

than the thermionic valves and certain switches, mechanical switches that had been used previously. And so that meant that you could use these devices in many more applications and they became very widely used first in the transistor radio and then later of course in television. And then when it was learned how to put different devices together onto the same silicon chip in what's called an integrated circuit, then you have the basis of modern microprocomputing.

So that was really the first wave, if you like, of the semiconductor revolution. That was based entirely on silicon. But there was another wave that came around about, so in our time during the last few decades. And this came about through the use of semiconductors, not only to process signals, but to make little diode lasers that were able to emit light. And that could be carried over long distances along optical fibers. and this sort of carrying, and that light carries information, and this is the basis of optical communications, and that led to the sort of explosion in communications technology

that's given us the broadband internet today. So can we develop these properties, Leslie Coen? Underneath this, are there any more properties I want to talk about super conduct? But can I just say, I think this is a sort of template, of an example of pure research by people like yourselves and people, leading to very quickly

to applications which have tremendous consequences, not only from the way I live a life, but for the wealth creation, for all sorts of things. It's a very interesting example of letting pure research run at universities. This isn't propaganda, this is just actually what's been going on, but the differences is so quickly being turned in,

the applications have happened so quickly in the last few years. Applications have happened quickly, but to defend fundamental science, it's a message. I'm not attacking fundamental science. I thought I was praising it. I must rephrase everything. I said on the last paragraph. You're absolutely are praising it,

and I would just like to build on it because it's something that perhaps our government does cannot afford to... We're not political on this programme. You're going too far. You're going beyond my... What did you say, Frank? Pay-way.

That's not possible, but not. Let me use an example from superconductivity. Camer. owns discovered superconductivity. At least he discovered one of the unique properties, its zero resistance, in 1911. So last year was the centenary. It took 22 years before Meisner discovered the other unique property. People weren't looking for it because it's not about it being a perfect superconductor. It's about it being, I'm going to say something now that I haven't explained, but it's a macroscopic quantum object.

It's entirely about quantum mechanics. And it took another 24 years till the 1950s, 1950s, till the full understanding from the quantum mechanics by Bardeen Cooper and Shrefer to set up the theory that would actually explain what people had been studying, using and already making money out of, by the way. of the basis of superconductivity. So there was 40 or 50 years of fundamental science before anybody really understood anything. In terms of applications, one whole area of applications relates to this macroscopic quantum nature of the superconductor

and its relation between current and field, magnetic field, and the fact you can use superconductors to detect absolutely tiny magnetic fields which has areas of interest, for example, in fetal heart detection of babies inside the world. Imaging there. Imaging. And the other area relates, and by the way, that was a Nobel Prize. Brian Josephson at the Cavendish discovered that in 1962, got a Nobel Prize in the 70s. and the other area relates to the zero resistance that good old Camling owns discovered.

And basically superconducting wire can carry huge currents in enormous magnetic fields. And so the really commercial benefit of superconductors are for, as you said at the beginning, magnetic resonance imaging because they provide huge magnets and other applications. Frank, Frank Close, can you tell us the relevance of superconductors to the large Hadron Collider? Well, in fact, there's two. One, just to pick up on the idea of the fundamental science, the understanding of superconductivity is what's indirectly led to all the ideas you now hear about the Higgs boson and so forth. These were very profound theoretical ideas picked up from trying to understand the phenomenon of superconductivity in its quantum nature,

taken over into the field of particle physics with all of these amazing predictions that people are now trying to test. So that shows you you can never tell quite where fundamental thought is going to lead you. How is it we're doing that? We are actually ironically using superconductivity to control the magnets at Large Hadron Collider. As was said earlier, conventional magnets, you have to power them up with electric currents, and you need very powerful currents to make these really big magnets work. if you've got conventional materials you're using a lot of heat, things get very, very hot. Superconductors, no resistance at all.

So if you've got superconducting materials, you can power the magnets up without disseminating all this heat. And so you use superconducting materials in designing and building the magnets that you use at CERN. Jenny Nelson, it seems to me that a very important application of semiconductors to the moment is solar cells. Could you tell us about those and how these discoveries over the last 100 years or so

have led to the position you're in now. Okay. Well, the position I'm in now is a researcher working on solar cells just for a point of sitting around this table. So in a solar cell, what you want to do is you want to turn light energy, so light from these photons, in the form of photons, it come from the sun. And the sun, of course, gives us a lot of different colours.

and we want to turn those into electrical energy. And actually, a semiconductor diode is almost the perfect environment in which to do that. That's because we've seen that light, so long as it has a colour equivalent to this energy gap or above, it can be absorbed in the semiconductor and it can push the electrons to a higher level where they're able to conduct. But then if you want it to do electrical work, you actually want all of those electrons to go in the same direction. So you need to have something built into the structure, which kind of tells the electrons which way to go.

And if you put in, so we know that a diode allows current to pass in only one direction, so if you build the diode into the structure, then it will force or encourage the electrons to go in the same direction. And then when you have a solar cell, you connect it to an external circuit, shine light on it,

it will generate a current, the current will come out into the external circuit, and you can then use it to do electrical work. And the first, I mean, we mentioned this was actually, it's interesting to sort of comment that it was actually one of the first, after the transistor, it was actually one of the first sort of applications that was discovered by these guys in Bell Labs in the USA and they presented it to the US government as being something

that might be interesting to exploit commercially. but there was no interest. So nobody at that time thought that it was worthwhile using this very, very pure silicon material to convert light energy into electricity. It was considered to be far too expensive, ever to be anything more than just a gimmick, a side interest. And now, of course, it's a gigantic interest industry.

So, Leslie, what other applications can we look forward to as we come towards the end of this programme. This is just a little hint. Well, I have unfortunately not had the opportunity to talk about all five of the Nobel Prizes associated with superconductivity, but the last one was Bednauts and Muller when they discovered what are known as high-temperature superconductors.

Unfortunately, they're still minus 150 degrees, not 120, 200 and whatever they were. Do the maths. Yeah, do the maths. And that still means that we have to cool them with liquid nitrogen. If we could find a room temperature superconductor, a lot of the electronics, the microelectronics industry and our computers would benefit from zero-resistance elements.

And, of course, power could be transmitted without any loss of energy, and that's terribly important, using the zero-resistance property. So its rim-temperature superconductivity would revolution. And revolutionise aspects of our modern world. Want a final word from you, Frank? I just think this whole story is a beautiful example of how fundamental ideas which are explored for their own intrinsic interest turn out years later to have applications that you never dreamed of.

It is, isn't it? I mean, that's one of the striking things for me. I mean, at the moment, I understand quite a lot of it. I mean, let's wait until Sunday. Anyway, thank you all very much for making it very clear for me at the time. And I think of a lot of people who are listening. Thank you, Leslie Cohen, Jenny Nelson and Frank Closer. Next week, by a miracle of joined-up programming.

I don't think we've ever done this before. We're going to do a program on Benjamin Franklin, which relates to this. We've never done that sort of thing. Thanks for listening. Thank you for listening to this Radio 4 podcast. If you've enjoyed it, you might like to try others like it, such as Start the Week or Thinking Aloud, which are both available from the Radio 4 website.