Video Transcript
In this video, our topic is
transistors. Transistors are part of many modern
electronic circuits. And this includes microprocessors
that we find in computing devices. There are many different types of
transistors, but in this lesson we’ll focus on two and see how they work.
Both of these transistor types
we’ll talk about are bipolar junction transistors. To better understand this term,
let’s begin by considering this word “junction.” From our knowledge of electrical
diodes, we can recall that a junction is formed whenever we take a semiconductor
material of one type, say, a p-type, here and join it up with the opposite type of
semiconductor material, n-type. A diode could either be formed by p
and then n second, or we could flip it around and have n and then p. Either way, at these interfaces
where the two different semiconductor material types meet, we have a junction. With these two types of diodes, p-n
and then n-p, we could see that they’re different just by their polarity.
If we hook up our p-n diode to an
electrical circuit with our voltage source oriented this way, we know that with the
positive terminal of our source pointing to the left, it will tend to send current
clockwise through the circuit. And with this particular diode
arrangement, a p-type semiconductor then joined to an n-type, charge can flow across
the junction between them. As a result of this, current can
exist all throughout the circuit. So as far as the polarity of our
diode goes, we can say that it’s forward biased. It’s arranged in such a way that
charge can successfully flow all the way through it.
But now let’s consider attaching
our circuit to the n-p diode instead. This is identical to p-n, but it’s
been switched around. Its polarity is reversed. And that means when effective
positive charge begins to flow clockwise through the circuit, in this case it’s
unable to cross through this junction from n-type to p-type. So the diode blocks the flow of
charge. And this means no current can exist
anywhere in the circuit. When a diode is arranged this way
relative to a voltage supply, we say that it’s reversed biased.
Now we bring all this up because
when we talk about transistors, we can think of them as being two diodes joined
together. Say, for example, that we take our
n-p diode and we attach it to the end of our p-n diode. We would get then this stack of
semiconductor materials, n then p then n-type. When we do this, we formed a
bipolar junction transistor. And specifically, this is called an
NPN transistor. Up to a point, we can think of this
transistor as two diodes joined together. We’ll talk in a moment about when
that doesn’t apply, but in many ways, thinking of a transistor this way helps us
understand how it works.
To see that, let’s look at how this
transistor works when we connect it up to a circuit. With our cell arranged this way, we
would expect conventional current to point in a clockwise direction. But we see that when that current
encounters our transistor, it runs into what is essentially a reverse-biased
diode. This means that charge can’t cross
the junction between these n- and p-types. Since charge doesn’t flow then
through this transistor, current can’t exist anywhere in the circuit. So right now, our transistor really
isn’t working as it should. All it’s doing is shutting down the
flow of charge.
But in general, a single transistor
can be used as a switch in a circuit or to amplify a current. For that to happen, we need to
modify our circuit a bit. We’re going to add a branch in our
circuit right here. And along with that, we’ll include
a second voltage supply. This supply will send conventional
current up into the p-type part of our transistor. And recall that if we consider this
current moving in this direction through our junction, then we can say that this
diode we’re effectively working with is forward biased. So because of that, charge can flow
through this part of our transistor. And therefore, current can exist in
this particular part of our circuit.
With all that charge flowing in
this smaller loop, if we then consider the current generated by what we called our
primary voltage supply, now with charge already flowing this way through part of our
transistor, the current coming from our primary supply actually is able to cross
this junction here than it normally wouldn’t be able to. We mentioned earlier that we can
think of transistors as two diodes joined together but that that’s only true up to a
point. Well, this is that point. Because if we were just thinking of
our n- and p-semiconductor types as a diode, then charge flowing across this
junction wouldn’t be possible.
But since we’re working with a
transistor and we already have charge flowing through this forward-biased portion of
it, under those conditions, charge actually can cross this barrier between the n-
and the p-type. And then once electrical charge has
made it that far, there’s nothing to stop it from continuing on all the way through
the transistor. And so then charge is able to flow
all the way around this outer, larger loop in our circuit.
Now in a typical transistor
circuit, the current here, that in effect turns the transistor on, is very small in
magnitude compared to the current here coming from our primary source. By this smaller current effectively
switching the transistor on, we can see then that we’re amplifying current through
the transistor as well as allowing current to exist at all in this portion of our
circuit.
Now that we’ve got our transistor
circuit working, notice that there are three wires that connect to the different
parts of the transistor. Each one of these wires or
connection points has its own specific name. Recalling that conventional current
travels clockwise through our circuit, this electric wire here is called the
collector. This is how our transistor collects
charge. Paired with this, on the other end
of our transistor, we have the emitter. This is where conventional current
or charge comes out. And then this last connecting wire
here that goes into the middle of our transistor is called the base. The base is what we supply with
this smaller amount of current from our secondary voltage source that then travels
out through the emitter and then allows charge to flow overall from collector to
emitter.
When we see a transistor like this
drawn in a sketch or a diagram, sometimes the collector, base, and emitter will just
be indicated by single letters. And note that we could also think
of these names as applying to the different regions of the transistor. When we see this, it can help us
remember what each letter stands for and why those names have been given.
So far, we’ve been looking at one
particular type of bipolar junction transistor, the NPN type. The second closely related
transistor we’ll consider is the PNP transistor. Like we might guess, that type of
transistor is formed by a p-type then an n-type then a p-type semiconductor joined
together. If we connect this kind of
transistor up to a circuit and if we once again consider this connection point the
collector, this one over here the emitter, and this one the base, then if we once
again connect up primary and then secondary voltage sources, for this transistor
circuit to work properly, we actually need to reverse the polarity of both of these
sources.
Set up like this, if we only
consider the effects of our primary voltage supply, we know that, oriented this way,
that will send current in a counterclockwise direction. When it reaches the transistor,
that charge will be able to cross over this p-to-n junction here. But then when it reaches this
effectively reverse-biased junction between the n- and the p-types, the charge will
be blocked. So then no charge will flow through
the transistor and, therefore, no current will exist in the circuit.
Just like before, though we have
this second voltage supply that sends current in a counterclockwise direction
through this smaller loop. And charge can indeed flow through
this forward-biased p-to-n portion of our transistor. When that happens, just like
before, it enables the passage of this current traveling through the larger outer
loop by letting the charge in that current pass over this junction here and continue
on out the collector of the transistor. And so by turning on our
transistor, we could say, by establishing current through the smaller loop, we allow
charge to flow all the way through the transistor and, therefore, all the way
through our larger circuit.
So these are the basic ideas behind
how PNP and NPN bipolar junction transistors work. Though these two types share
similarities, notice that the direction of current through them is opposite. In the NPN transistor, conventional
current flows into the base and then out the emitter, which allows what we could
call the main circuit current to enter through the collector and leave through the
emitter. On a PNP transistor though,
everything is reversed. The conventional current coming
from the base branch of our circuit enters the transistor through the emitter
portion. And this allows the main circuit
current, we could call that the current in the larger outer loop, to travel from
emitter to collector through the transistor.
So depending on whether we’re
talking about an NPN or PNP transistor, the direction of conventional current flow
through the transistor varies. Knowing all this, let’s get some
practice with these ideas through an example.
A PNP transistor is connected to a
direct current source, as shown in the diagram. The two p-regions are
identical. Which of the regions of the
transistor is the collector region? Which of the regions of the
transistor is the emitter region?
Looking at our diagram, we see our
PNP transistor connected up to an electrical circuit. The three regions of this
transistor called P one, N, and P two describe a specific type of semiconductor
material, either p-type or n-type. We’re told that the two p-regions
in this transistor, P one and P two, are identical. And seeing how this transistor is
arranged in this electrical circuit, we first wanna answer the question of which of
these three regions in the transistor is the collector region.
Now, this term “collector” refers
to a connection point into the transistor. And we see from our diagram there
are three of those, this point right here, this one, and then this one. To help us figure out which of
these connection points attaches to the collector region, we can recall that in
general a PNP transistor has a collector region, a base region, and an emitter
region. And note that the collector and the
emitter both correspond to p-type semiconductor regions.
So considering the PNP transistor
in our sketch, we know that the answer to this first question, which of the regions
is the collector region, is either going to be P one or P two. That is, one end of this transistor
is the collector and the other is the emitter. But which one is it? Is P one the collector and P two
the emitter? Or is it the opposite?
To answer this question, we’ll need
to look at the way that conventional current travels through the transistor. Because of the polarity of our
voltage supply, we know that conventional current will point in a counterclockwise
direction all through this circuit. And note that we’re assuming our
transistor has been switched on so that current can indeed exist all through this
loop. Now that we knew this, the real
question, we could say, that this first part of our question is asking is, for a PNP
transistor, does conventional current enter through the collector or does it enter
through the emitter?
Based on the names of these
regions, we might expect conventional current to enter through the collector and
leave through the emitter. As it turns out, though, that’s
only true for an NPN transistor. But here we have what we could call
the opposite type, PNP. That fact means that the direction
of conventional current travel through our transistor actually moves, as we’ve drawn
it, from right to left, from the emitter to the collector. So then it’s the second p-type
region that our current reaches which is the collector in a PNP transistor. And as we look at the way charge
will flow through this transistor in our diagram, we see that that second region is
P two. Since that is the last region of
our transistor that charge passes through as it moves in this circuit, we know it’s
the collector region of our transistor.
Now that we figured this out,
finding the answer to the second part of our question is simpler. Here we want to identify which
region of the transistor is the emitter region. We’ve seen that in a PNP
transistor, charge moves from the emitter to the collector. And therefore, in our diagram, the
first region of our transistor that conventional current encounters is the emitter
region. That’s P one. And that’s our answer to “Which of
the transistor regions is the emitter region?”
Let’s take a moment now to
summarize what we’ve learned about transistors. In this lesson, we saw that
transistors are electrical components that are made of n-type and p-type
semiconductor materials joined together. Two common types of transistors,
both called bipolar junction transistors, are NPN and PNP. When an NPN transistor is set up in
a circuit, charge is first to set up to flow through the forward-biased p-n junction
of the transistor here. This means that charge flows into
what is called the base region of the transistor and out the emitter region. And this allows charge in the
larger circuit loop to flow in to the collector region of the transistor and then
out the emitter.
With a PNP transistor, on the other
hand, when this transistor is in a circuit, all these current directions are
reversed. First, charge flows through the
forward-biased p-n junction here in a counterclockwise direction. And then this allows charge to flow
this way from emitter to collector through the transistor overall. So then in an NPN transistor
overall current travels from collector through emitter, while in a PNP transistor it
travels from emitter to collector. This is a summary of
transistors.