Video: Diodes

In this video, we will learn how to describe how diodes work and identify materials used in their construction.

14:18

Video Transcript

In this video, we will be looking at an electrical circuit component known as a diode. We will be looking at the function of a diode in an electrical circuit as well as what it’s made from.

So let’s start by thinking about what a diode actually does. A diode is an electrical circuit component that allows a current through it in one direction but not in the opposite direction. This is the circuit symbol for a diode. And it’s actually one of the more friendly circuit symbols because the arrow in this symbol shows us the direction in which conventional current is allowed to flow through the diode. And remember, by the way, conventional current is made up of positive charges.

So, for example, if we were to take this diode and connect it in series with a cell and a resistor, then in this orientation of the cell, conventional current will flow away from the positive terminal and through the diode in the allowed direction through the resistor and then back round the circuit until we complete the circuit when we get to the negative terminal of the cell. In other words then, in this orientation of the cell and the diode, a clockwise current made up of positive charges would be set up in the circuit.

However, if we were to think about the flow of electrons, which are negatively charged particles, then those will be flowing away from the negative terminal of the cell going counterclockwise through the resistor and through the diode. And this is actually the allowed direction for negatively charged particles to flow. Because, remember, the arrow in the diode only shows the direction in which positive charges can flow. And, hence, negative charges can flow the other way through the diode.

However, if we were to reverse the polarity of the cell so that the positive terminal is now here and the negative terminal is here, and then if we think about conventional current, the cell is now trying to set up a current that flows in this direction, away from the positive terminal through the resistor and arriving at the diode. However, a diode will not allow positive charges to flow in that direction through it. And, hence, there will be no current in this circuit. If we equivalently think about negative charge flows, the flow of electrons, then the cell is trying to push the electrons this way. However, negative charges cannot go in this direction through the diode. And so, as we realized before, there will be no current in this circuit.

So that is the functionality or the behavior of an ideal diode. And, yes, in this circuit, we are considering ideal components. But as we will see shortly, real diodes don’t quite behave like ideal diodes. Just like, for example, how real wires actually have some resistance whereas we model ideal wires in our circuit diagrams to have zero resistance. So real circuit components don’t behave quite like their ideal counterparts. And the diode is no exception to this. But let’s hold on to that thought and come back to that in a moment.

First, though, let’s think about an ideal diode, once again, in our ideal circuit. Let’s now get rid of the resistor. And, instead, put an ammeter in that position as well as a voltmeter in parallel with the diode. And let’s also take our cell and turn it into a variable voltage source. So the whole point of this is that we’ll vary the voltage put out by the variable voltage source and look at the voltage across the diode as a response to this as well as the current in the circuit. And because the ammeter is in series with the diode, it will therefore measure the current through the diode.

Now, let’s decide upon a convention that a current flowing in the clockwise direction is flowing in the positive direction. And, hence, any potential difference that sets up a current in the clockwise direction is a positive potential difference. Let us then set up a pair of axes with the current measured by the ammeter here on the vertical axis and the potential difference 𝑉 measured across the diode by this voltmeter. And we put that potential difference on the horizontal axis. So now, in this orientation of the cell, regardless of the magnitude of the voltage put out by the cell, we know that the cell is trying to set up a current in this direction. And that’s conventional current.

So the cell is trying to push positive charges in this direction. However, the diode does not allow this. And, in fact, an ideal diode will do this regardless of the magnitude of the potential difference put out by the cell. Now, remember, we said earlier that any voltage trying to push a current in the clockwise direction is a positive voltage. Well, in this case, our cell is trying to push a current, a conventional current, in the negative direction, the counterclockwise direction. And for an ideal diode, if we start at zero voltage and increase the voltage in the negative direction, what we’ll see is that the current is zero regardless of that negative voltage. Because no matter how much this cell tries to push a conventional current in this direction, the diode will not allow it. And so, our 𝐼–𝑉 graph is a flat line for all negative voltage values because the current is zero.

However, if we now reverse the polarity of our variable cell and slowly increase the voltage in this direction, then what we see is that the cell is now trying to set up a conventional current flowing clockwise in our circuit. Well, in that situation, as we increase the voltage on our variable voltage source, the voltmeter measures an increasing voltage, because that’s the voltage across the diode as well. And what we expect is that the current will start to flow immediately as soon as the voltage increases above zero. Moreover, an ideal diode, when in the correct orientation relative to the cell, will actually act like a closed switch.

And so, at this point, the only components we’ve got in our circuit are the cell, the closed switch, the voltmeter, and the ammeter. This basically means that a massive current can flow because there’s no resistance to this current. And, hence, what we will see on a current-versus-voltage graph is that as soon as the voltage increases above zero, we get a massive value for the current. So what we’ve seen is that if the orientation of our diode is correct relative to our cell — in other words, if the diode actually allows the current that the cell is trying to set up — then the diode basically acts like a closed switch. But if we reverse the polarity of the cell and the cell tries to push a counterclockwise current through the circuit, then the diode actually acts like an open switch. And that actually prevents any current from existing in the circuit.

So that is the behavior of an ideal diode. And this graph shows its 𝐼–𝑉, or current–voltage, characteristics. In other words, this graph shows us what we’d expect to see when we change the potential difference across the diode and measure the current through that diode. However, as we mentioned earlier, real diodes don’t quite behave like ideal diodes. So here are the 𝐼–𝑉 characteristics of an ideal diode once again. And here are the 𝐼–𝑉 characteristics of a real diode, so quite different to what we’re expecting.

Let’s look at this section first of all. We can see that for very high negative values of the voltage, a current in the negative direction does actually exist. In other words, at very high negative voltages, a real diode will break down and will actually allow a current to pass through it in the, quote, unquote, wrong direction. In other words, if we come back to our circuit from earlier and set up the polarity of the cell such that it’s trying to push a current in the wrong direction through the diode, what our real diode 𝐼–𝑉 graph tells us is that if the potential difference across the diode is large enough, then eventually a current in the counterclockwise direction will be allowed to flow. And so, at very high negative voltages, the diode actually breaks down.

Now, at smaller negative voltage values, an ideal diode would allow zero current through the circuit whereas in this case we do have a very small current passing through the circuit. So even with a reverse polarity cell, there is a very tiny current passing counterclockwise through the real circuit. And the diode does allow this. Now let’s think about what happens when the voltage becomes positive. In other words, we reverse the polarity of the cell once again so that the cell is trying to set up a current in the direction that the diode will allow current to pass through it. And, remember once again, this current that we’re talking about is conventional current.

Well, with an ideal diode, what we expect to see is that as soon as the voltage becomes anything even slightly larger than zero volts, a massive current is set up in the circuit. Because remember that ideal diode acts like a closed switch in the situation. However, the real diode behaves slightly differently. What we see is that there is a certain minimum voltage that must be applied before any current is allowed to pass through the circuit. Now, this voltage is known as the threshold voltage, which we’ll call 𝑉 subscript 𝑡. Which, for an ideal diode, is actually zero volts because anything above this, and a current is immediately set up in the circuit. So those are the main differences between the 𝐼–𝑉 characteristics of an ideal diode and real diode.

Now, it’s all well and good thinking about diodes as circuit components that behave in this particular way. But we can ask the question, what are diodes actually made from, what materials? Well, diodes are most commonly made from semiconductor materials, such as silicon. Now, a semiconductor material is a material that’s not quite as good at conducting electricity as a conductor. But it’s a much better conductor than an insulator. In other words, a semiconductor’s conductivity lies somewhere between the conductivity of an insulator and the conductor. And silicon is an example of a semiconductor. A silicon atom has four electrons in its outermost shell. Which means that it can form bonds with four other silicon atoms.

So, for example, if we considered this silicon atom, then we see that it’s bonded with this one, this one, this one, and this one. And the net result of this is that in the outer shell of that silicon atom, there now one, two, three, four, five, six, seven, eight electrons. In other words, a complete outer shell. Four of those eight electrons come from this silicon atom itself. And the other four come from the four silicon atoms that it’s bonded with. Now, silicon is a semiconductor. Because if we were to take this crystal of silicon and heat it, then some of the electrons in these bonds would be able to escape to high energy levels. And so, what’s left are little gaps where there’s a space for an electron. And this means that other electrons can then come fill these gaps. And because electrons are charged particles, this means that there can be a flow of charged particles — or, in other words, a current — in silicon.

However, by itself, the conduction properties of silicon aren’t quite good enough. And so, what we can do is to replace some of these silicon atoms with atoms of another element. One such element is boron. Boron only contains three electrons in its outermost shell. And so, we could replace one of these silicon atoms, let’s say this one, with a boron atom in a process known as doping. Now, because, in this position, we have a boron atom instead of a silicon atom, that boron atom has one fewer electron in its outer shell to provide for bonding. And this means that without even having to heat up our silicon crystal, we’ve now got a hole where an electron would’ve been if this atom was silicon. That hole can then be occupied by other electrons which leave behind holes when they jump to this position.

Now, boron being a trivalent atom, which means that it has three electrons in its outer shell, result in a doped silicon crystal that now has more holes than it would have otherwise. And these hoes are considered to be positively charged because they’re at the absence of an electron which is negative. And so, the absence of a negative charge can be thought of as a positive charge. And, hence, this type of doped silicon crystal is known as a p-type or positive-type semiconductor. However, if we were to dope our silicon crystal, instead, with a pentavalent atom — so that’s an atom containing five electrons in its outer shell as opposed to the four of silicon, and an example of a pentavalent atom is phosphorus — then what we’d see is that phosphorus forms four bonds with the silicon atoms around it. But, now, there’s an excess electron from the phosphorus that actually becomes free to move around inside the crystal.

This means that a pentavalent atom has an extra electron compared to a silicon atom. And that extra electron is free to move around. Which means that this negatively charged particle, this electron, can move and could thus form a part of a current flow. Therefore, this also increases the conductivity of our silicon crystal. And a crystal doped with a pentavalent atom is known as an n-type or negative-type semiconductor. This is because it provides an excess of negatively charged particles or electrons.

So we’ve looked at p-type semiconductors and n-type semiconductors. But what does that have to do with diodes? Well, it turns out that if we take a p-type semiconductor and an n-type semiconductor and join them in the middle, then this set-up acts like a diode. In other words, connecting our cell in this orientation allows a flow of conventional current in the clockwise direction in our circuit as we’ve drawn it. Whereas if we were to switch the polarity of the cell, then the p-type semiconductor and the n-type semiconductor together, known as a p-n junction, would not allow a counterclockwise conventional current to be set up in the circuit. And thus the ammeter would measure a current of zero amps. So now that we’ve had a look at the functionality of a diode as well as what a diode is made from, let’s get some practice while looking at an example question.

Which of the following correctly describes a diode? A) A diode is an electronic component that emits light with a very high efficiency. B) A diode is an electronic component with a resistance that changes depending on how much light is incident upon it. C) A diode is an electronic component that only allows a current to flow in one direction through it. D) A diode is an electronic component that can be used to amplify signals. E) A diode is an electronic component with a resistance that changes depending on ambient temperature.

Okay, so in this question, out of the options A to E, we’ve been asked to select the one that describes a diode. To answer this, it might help to remember the circuit symbol of a diode. This is how we draw a diode in a circuit diagram. And this diagram is particularly helpful to us because, in this diagram, we can see that there’s a little arrow. And we can recall that this arrow signifies the direction in which conventional current is allowed to flow through a diode. In other words, conventional current, that’s current made up of positive charges, can flow in that direction but is not allowed to flow in the other direction. And that is the functionality of a diode. In other words then, a diode is an electronic component that only allows a current to flow in one direction through it. Hence, that is the answer to our question.

Very quickly looking at the other options though, we can see that the description in option A is that a diode is an electronic component that emits light with a very high efficiency. Well, that type of component is actually a very specific kind of diode, specifically, a light emitting diode or LED. And LEDs do actually emit light with very high efficiency. However, this is not a good description of a diode in general because not all diodes are light emitting diodes. And, hence, we haven’t picked option A as the answer to our question.

Option B says that a diode is an electronic component with a resistance that changes depending on how much light is incident upon it. Well, that sounds more like a description of a light-dependent resistor or an LDR. And so, that’s not the answer to our question either. Option D says that a diode is an electronic component that can be used to amplify signals. Well, that sounds very much like an amplifier, which are often made up of transistors as well as others circuit components of course. And so, this does not sound like a description of a diode. And, finally, option E says that a diode is an electronic component with a resistance that changes depending on ambient temperature. Well, that circuit component is known as a thermistor, or sometimes a temperature-dependent resistor. And, hence, option E is also not the answer to our question. So now that we’ve looked at this example, let’s summarise what we’ve talked about in this lesson.

We saw, firstly, that diodes are electronic components that allow current flow through them in one direction but not in the opposite direction. We also saw that this is the circuit symbol for a diode, which is helpful because the arrow shows us the direction in which conventional current can flow through the diode. Next, we saw that the 𝐼–𝑉 characteristics of an ideal diode look like this whereas those of a real diode look like this. We also saw that semiconductors, such as silicon, can be doped to form p-type, or positive-type, and n-type, or negative-type, semiconductors.

And, finally, we saw that diodes are commonly made from p-type silicon, atoms such as boron which has three electrons in its outer shell replace some of the silicon atoms in the crystal, and also n-type silicon, where a pentavalent atom such as phosphorus will replace some of the silicon. And when we join a p-type semiconductor and an n-type semiconductor together in the form of a p-n junction, then this displays the behavior of a diode.

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