Video Transcript
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?”
