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  • Sometimes, in the history of engineering, physicists make their way into the story.

  • Consider, for example: John Bardeen.

  • You might not have heard of him, but Bardeen is the only person in all of history to win the Nobel Prize in Physics twice.

  • He shared the prize once in 1956 and again in 1972.

  • Both times, he was involved in discoveries that made clever use of materials for electrical engineering.

  • The first prize was for his work on the development of the transistor,

  • and the second was for describing a way to allow certain materials to conduct electricity with no resistance, called superconductivity.

  • Superconductivity is tricky to achieve, but it might allow us transport electricity with nearly perfect efficiency in the future.

  • Transistors, meanwhile, are electrical components that have already revolutionized society.

  • They form the basis of all modern computing.

  • And at their heart are semiconductors.

  • [Theme Music]

  • As the name implies, semiconductors are in between materials that conduct electricity and materials that are totally non-conducting.

  • A classic example would be silicon, which is so commonly used as a semiconductor that Silicon Valley was named after it.

  • As we'll see, semiconductors have been transforming the face of technology for decades.

  • On its own, silicon doesn't conduct electricity very well.

  • It has no free charges, like free electrons, in it to carry a current.

  • But you can alter the structure of silicon on an atomic level to change that.

  • First, you replace a few of the atoms in a layer of silicon with phosphorus atoms.

  • Those phosphorus atoms each carry one more electron than a silicon atom would have,

  • which introduces more negatively-charged electrons into the material.

  • Those extra electrons aren't bound to any of the silicon atoms and are therefore free, negative charges.

  • For that reason, we call this an N-type semiconductor – “N” for negative.

  • On the other hand, if you switch some of the silicon atoms with boron atoms, it creates a relative lack of electrons in the material.

  • The boron atom has one less outer electron than the silicon atom it's replacing.

  • The places where those electrons are absent are called holes.

  • The lack of negative charge creates regions of free positive charge, so we call this arrangement a P-type semiconductor.

  • Holes can move around and be transported in a material, just like the absence of water in a sealed container – a level, for example

  • forms a bubble whose location you can keep track of.

  • You can think of holes as effectivelypositive chargesthat can be filled by the presence of an electron.

  • When that happens, the space left behind by that electron creates a new hole.

  • Now, on their own, those P- and N-type semiconductors aren't all that exciting.

  • Unlike pure silicon, which is an insulator, they'll be weakly conducting because they now have free charges moving around.

  • But things get really interesting when you put them togetherlike if you have both P and N type conductors sandwiched together in a circuit.

  • Normally in electrical circuits, the electrons flow from the negative terminal to the positive.

  • Remember that the current is defined to flow in the opposite direction, so it travels from positive to negative.

  • One thing you can do by sandwiching both types of semiconductors is to stop the current in a circuit from flowing altogether.

  • If you put the N-type semiconductor on the positive terminal side, and the P-type semiconductor on the negative terminal side, together they stop the current,

  • even though on their own, they'd each be weakly conductive.

  • It works because each semiconductor's extra charges are the opposite of the terminal it's next to.

  • All the negatively charged electrons in the N type semiconductor are drawn towards the positive terminal of the circuit.

  • And all the holes in the P type semiconductor are drawn to the negative terminal of the cell.

  • Since the effective charges are being pulled away from the area between the semiconductors,

  • you end up with a gap between the two plates where charge can't be transported across, called a depleted region.

  • Since there are no free charges to transport a current across that gap, an electrical current can't pass through it.

  • So if you arrange P- and N-type semiconductors this way around, you can stop a flow of current.

  • But, if you arrange them the other way around, with the P-type near the positive terminal and the N-type near the negative terminal,

  • now the electrons and the holes are drawn towards each other.

  • In this case, the extra electrons from the N-type fill the holes in the P-type, and the new holes spring up where the electrons used to be.

  • This kind of cascade can happen throughout the entire circuit, again and again until you have electrons flowing much as before.

  • In other words, in this arrangement the current can now travel through where the N and P layers meet.

  • The purpose of N and P type semiconductors put together is the simplest form of what we call a diode, which is basically a one-way enforcer of electrical current.

  • It allows current to flow in one direction, but stops it from flowing in the opposite direction.

  • It just depends on how you insert the semiconductors relative to the terminals of your voltage supply

  • whether that's a battery, an electrical outlet, or something else.

  • Being able to control the flow of a current can be really useful.

  • For example, you might have an alternating current, or AC signal flowing through a circuit,

  • where the direction of the current changes back and forth.

  • But many electrical components need a direct current, or DC, with a flow of current in only one direction.

  • In the right arrangement, diodes can be used to convert a wavy, AC current into nice simple DC current.

  • The flow of charge in the DC part of the current always goes the same way,

  • and the positive and negative ends of the output remain the same, like a battery.

  • So, diodes are handy for controlling the direction of a current.

  • But you can do even more if you put three semiconductors together in a sandwich.

  • You have two options for this type of sandwich: P-N-P, or N-P-N.

  • In both cases, the middle layer effectively creates a diode with each of the outer layers,

  • with each diode allowing current to flow in the opposite direction.

  • This may not sound terribly useful, because together, the three semiconductor layers are restricting flow both ways.

  • Unless, that is, you add a second current.

  • Let's say you have an N-P-N sandwich connected to a battery.

  • The current can't flow through it, because you have a positive terminal hooked up to an N layer.

  • The electrons in that layer will be attracted to the positive terminal, while the holes in the P layer are attracted to the other N layer.

  • So you end up with a depleted region between them, and the current is going nowhere.

  • But here's the incredible thing.

  • If you apply just a small current that flows from the middle plate to the N layer on the same side as the first battery's negative terminal,

  • the electrons moving into the P layer fill the depleted region between it and the other N layer.

  • So that gap the electrons couldn't cross before disappears, and the original larger current is free to flow across the whole sandwich.

  • In other words, you've created an electrical switch – a sort of gatewaythat requires just a tiny current to control the flow of a larger current.

  • And it works for P-N-P arrangements, too.

  • This arrangement of semiconductors, that might seem so functionless at first, is a transistor.

  • And the fact that it allows you to control how current flows in a circuit makes it one of the most important components of the electronic age.

  • Since transistors use smaller currents to influence the on or off states of the larger currents flowing through the wire,

  • they form the basis of the binary system of 1s and 0s that computers rely on.

  • All the marvels of computers and computer chips, including your ability to watch this video,

  • depend on semiconductors and the transistors we make from them!

  • So, that's how materials like semiconductors can direct the flow of electrical power.

  • But semiconductors can be used to generate electrical currents, too!

  • And that ability has allowed us to take advantage of an incredibly useful source of clean, renewable energy.

  • To see how this works, let's go back to a simpler diode set up, with a P-type and N-type semiconductor put together.

  • This time, you don't connect the two sides to a power supply.

  • Instead, you attach it to a device you want to power, like a small electrical motor.

  • Remember, the N-type will have an abundance of free negative charges and the P-type will have an abundance of free holes.

  • There's no voltage being applied across the junction between the two types, so the electrons of the N-type will naturally fill the holes in the P-type.

  • This creates a depleted region at the interface between the two semiconductors.

  • There are no free charges because the electrons become weakly bound to atoms when they fill the holes that were in the P-type.

  • The N-type has a small region with some positively charged atoms from the absence of those electrons,

  • while the P-type has a small region with some negative charge from those extra electrons it picked up.

  • These opposite charges set up an electric field across the gap.

  • If there were any free electrons in this field, they'd be driven away from the negatively charged region in the P- type, towards the positive region set up in the N-type.

  • So that's our setup.

  • Now, how do you get energy from this?

  • On its own, it's not going to do an awful lot.

  • But electrons in a material can respond to light.

  • When light hits them, the electrons interact with the light and can even absorb some of its energy.

  • If the bound electrons in the P-type absorb energy from the light shining on the material,

  • they can get just enough energy to stop being bound to atoms and become free charges!

  • Remember that the charges across the gap set up an electric field.

  • Electric fields apply a force to free electric charges, so an electron freed from the extra energy it got from the light is now driven by that electric field into the N-type.

  • That leaves a hole in the P-type waiting to be filled,

  • but the electrons can't flow back against the electric field; the forces push it the other way around.

  • Instead, the N-type's extra electron will flow all the way around the circuit, through the device, delivering electrical power.

  • That's a solar cell!

  • With the right arrangement of semiconductors, it allows you to generate electricity from light.

  • These kinds of cells are exactly what form the basis of solar panels.

  • With semiconductors, and silicon in particular, you can create electrical power from sunlight.

  • And I think you'll agree, that's a pretty bright idea.

  • In this episode, we looked at silicon, and how introducing small amounts of other elements

  • allow silicon layers to conduct currents, turning them into semiconductors.

  • We saw how putting two different types – N and P semiconductorstogether gave us electrical components like diodes, transistors, and solar cells.

  • Next on our tour of materials engineering, we'll be going super tiny as we explore the world of nanomaterials.

  • Crash Course Engineering is produced in association with PBS Digital Studios.

  • If you want to keep exploring the world around us, check out Reactions: a show that uncovers

  • the chemistry all around us, and answers the burning questions you didn't know were chemical

  • - from whether gum really stays in your stomach to why bacon smells so good.

  • Check out Reactions and subscribe at the link below.

  • Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people.

  • And our amazing graphics team is Thought Cafe.

Sometimes, in the history of engineering, physicists make their way into the story.

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シリコン、半導体、太陽電池。クラッシュ・コース・エンジニアリング #22 (Silicon, Semiconductors, & Solar Cells: Crash Course Engineering #22)

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    林宜悉 に公開 2021 年 01 月 14 日
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