Video Transcript
An NPN transistor is connected to a
direct-current source, as shown in the diagram. Which of the two identical
n-regions of the transistor is forward biased? N one, N two, both regions are
forward biased.
The diagram shows an NPN transistor
with two n-type regions sandwiching one p-type region. From the question, we know both of
the n-type regions are identical, which means we don’t have to worry about
variations inside of the transistor itself. What we are trying to determine is
which of these two n-type regions is forward biased. Forward biasing and its counterpart
reverse biasing are the two possibilities when a PN junction is connected to a
direct current source. Here, we’ve drawn a picture of a
forward-biased and a reverse-biased PN junction. Note that the only difference
between these pictures is the orientation of the power supply, that is, which
terminal is connected to the p-type semiconductor and which terminal is connected to
the n-type semiconductor.
A forward-biased junction has the
positive terminal of the power supply connected to the p-type semiconductor and the
negative terminal of the power supply connected to the n-type semiconductor. We call this configuration forward
biased because charge can flow freely across the PN junction. In other words, the PN stack
conducts. When a junction is reverse biased,
the positive terminal of the power supply is connected to the n-type semiconductor
and the negative terminal of the power supply is connected to the p-type
semiconductor. In this configuration, charge
cannot flow freely across the PN junction. The stack is insulating, and since
this behavior is opposite to that of a forward-biased junction, we call it reverse
biased.
Looking back at our diagram, we see
that the positive terminal of the power supply is on the left and the negative
terminal is on the right. The transistor also consists of two
PN junctions, one with N one on the left and P on the right and the other with P on
the left and N two on the right. Starting with the PN junction on
the left, we see that N one is connected to the positive terminal of the dc current
source, which means using our definition of reverse and forward bias, this is a
reverse-biased PN junction. Looking at the other PN junction,
we see that N two is connected to the negative terminal of the dc current source,
which means that this region is forward biased.
And there’s our answer. Of the regions in our transistor, N
two alone is the forward-biased n-type region.
