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How does a transistor work?

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In this phone, there are nearly 100 million transistors, in this computer there's over
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a billion. The transistor is in virtually every electronic device we use: TV's, radios,
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Tamagotchis.
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But how does it work?
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Well the basic principle is actually incredibly simple. It works just like this switch, so
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it controls the flow of electric current.
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It can be off, so you could call that the zero state or it could be on, the one state.
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And this is how all of our information is now stored and processed, in zeros and ones,
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little bits of electric current. But unlike this switch, a transistor doesn't have any
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moving parts. And it also doesn't require a human controller. Furthermore, it can be
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switched on and off much more quickly than I can flick this switch. And finally, and
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most importantly it is incredibly tiny. Well this is all thanks to the miracle of semiconductors
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or rather I should say the science of semiconductors.
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Pure silicon is a semiconductor, which means it conducts electric current better than insulators
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but not as well as metals. This is because an atom of silicon has four
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electrons in its outermost or valence shell. This allows it to form bonds with its four
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nearest neighbours,
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Hidey ho there! G'day
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Wasaaaaap!?
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So it forms a tetrahedral crystal.
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But since all these electrons are stuck in bonds, few ever get enough energy to escape
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their bonds and travel through the lattice. So having a small number of mobile charges
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is what makes silicon a semi-conductor.
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Now this wouldn't be all that useful without a semiconductor's secret weapon -- doping.
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You've probably heard of doping, it's when you inject a foreign substance in order to
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improve performance.
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Yeah it's actually just like that, except on the atomic level.
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There are two types of doping called n-type and p-type. To make n-type semiconductor,
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you take pure silicon and inject a small amount of an element with 5 valence electrons,
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like Phosphorous.
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This is useful because Phosphorous is similar enough to silicon that it can fit into the
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lattice, but it brings with it an extra electron. So this means now the semiconductor has more
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mobile charges and so it conducts current better.
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In p-type doping, an element with only three valence electrons is added to the lattice.
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Like Boron. Now this creates a 'hole' - a place where there should be an electron, but
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there isn't. But this still increases the conductivity
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of the silicon because electrons can move into it.
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Now although it is electrons that are moving, we like to talk about the holes moving around
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-- because there's far fewer of them. Now since the hole is the lack of an electron,
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it actually acts as a positive charge. And this is why p-type semiconductor is actually
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called p-type. The p stands for positive - it's positive charges, these holes, which are moving
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and conducting the current.
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Now it's a common misconception that n-type semiconductors are negatively charged and
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p-type semiconductors are positively charged. That's not true, they are both neutral because
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they have the same number of electrons and protons inside them.
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The n and the p actually just refer to the sign of charge that can move within them.
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So in n-type, it's negative electrons which can move, and in p-type
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it's a positive hole that moves. But they're both neutral!
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A transistor is made with both n-type and p-type semiconductors. A common configuration
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has n on the ends with p in the middle. Just like a switch a transistor has an electrical
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contact at each end and these are called the source and the drain. But instead of a mechanical
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switch, there is a third electrical contact called the gate, which is insulated from the
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semiconductor by an oxide layer.
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When a transistor is made, the n and p-types don't keep to themselves -- electrons actually
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diffuse from the n-type, where there are more of them into the p-type
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to fill the holes.
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This creates something called the depletion layer. What's been depleted? Charges that
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can move. There are no more free electrons in the n-type
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-- why? Because they've filled the holes in the p-type.
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Now this makes the p-type negative thanks to the added electrons. And this is important
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because the p-type will now repel any electrons that try to come across from the n-type.
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So the depletion layer actually acts as a barrier, preventing the flow of electric current
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through the transistor. So right now the transistor is off, it's like an open switch, it's in
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the zero state.
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To turn it on, you have to apply a small positive voltage to the gate. This attracts the electrons
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over and overcomes that repulsion from the depletion. It actually shrinks the depletion
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layer so that electrons can move through and form a conducting channel.
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So the transistor is now on, it's in the one state.
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This is remarkable because just by exploiting the properties of a crystal we've been able
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to create a switch that doesn't have any moving parts, that can be turned on and off very
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quickly just with a voltage, and most importantly it can be made tiny.
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Transistors today are only about 22nm wide, which means they are only about 50 atoms across.
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But to keep up with Moore's law, they're going to have to keep getting smaller. Moore's Law
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states that every two years the number of transistors on a chip should double.
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And there is a limit, as those terminals get closer and closer together, quantum effects
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become more significant and electrons can actually tunnel from one side to the other.
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So you may not be able to make a barrier high enough to stop them from flowing.
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Now this will be a real problem for the future of transistors, but we'll probably only face
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that another ten years down the track. So until then transistors, the way we know them,
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are going to keep getting better.
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Once you have let's say three hundred of these qubits, then you have like two to the three
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hundred classical bits. Which is as many particles as there are in
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the universe.

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