A transistor is an electronic device that can be used as an amplifier or as an electronic
switch. Its ability to amplify a signal or to switch high power loads using a small signal
makes it very useful in the field of electronics. There are two basic types of transistors,
the bipolar junction transistor, or BJT, and the field-effect transistor, or FET. In this
video, we will just focus on the bipolar junction transistor and discuss its basic structure
and operation.
The bipolar junction transistor is manufactured with three semiconductor regions that are
doped differently. If we’ve already lost you with that last sentence, please go check
out some of our other tutorials on the basics of semiconductors, as it’ll make this a
lot easier to understand. These three regions that are doped differently are known as the
base, collector, and emitter. The base region is lightly doped and is very thin compared
to the collector and emitter regions. The collector region is moderately doped while
the emitter region is heavily doped.
Bipolar junction transistors can be an npn or a pnp type. The npn type consists of two
n regions separated by a p region. The base region is the p-type material while the collector
and emitter regions are n-type materials. In pnp type, the transistor consists of two
p-type regions, the collector and emitter, separated by an n-type base region. Regardless
of the type, a BJT has two pn junctions that must be correctly biased with an external
DC voltage to operate properly. One of these junctions is called the base-emitter junction,
connecting the base and emitter regions and the other one is the base-collector junction,
connecting the base and collector regions.
In order for a bipolar junction transistor to operate as an amplifier, its base-emitter
junction must be forward-biased while the base-collector junction is reverse-biased
- please note that this means that an npn transistor and a pnp transistor are backwards
compared to each other. And, as mentioned earlier, the emitter region is heavily doped.
So in an npn transistor, the n-type emitter region has a very high density of free electrons
while in a pnp transistor the p-type emitter region has a very high density of holes.
At this point, I’d like to remind you that current and electron flow are backwards, which
may cause confusion. Since the base-emitter junction is forward-biased, free electrons
from the emitter region easily cross the base-emitter junction and go into the very thin and lightly
doped p-type base region. The p-type base region is just lightly doped, which means
that, it doesn’t have that many holes in it. In this case, only a small percentage
of the free electrons from the emitter region can recombine with the holes in the base region.
The small number of free electrons from the emitter region that recombined with the holes
in the base region move through the base region as valence electrons. But when they leave
the base region and move through the metallic base lead, they become free electrons and
produce the external base current, which then goes out through the metallic lead, into the
external circuit, and then, eventually, return to the emitter region.
The free electrons that entered the base region but didn’t recombine with the holes move
toward the reverse-biased base-collector junction. Since the collector region is connected to
the positive side of the external bias voltage, the free electrons are attracted to the positive
side and are swept across into the collector region. They exit the collector region and
and also move through the metallic collector lead, into the circuit, and return into the
emitter region. So in this case, we know that the emitter current is the sum of the
base and collector currents. Therefore, the emitter current is slightly greater than the
collector current.
The operation inside a pnp transistor is very similar to the npn type. But the roles of
the electrons and holes are swapped. The external bias voltages and the current directions are
all reversed.
If we compare the direction of the currents of an npn and a pnp transistor using conventional
current flow, we’ll see that the flow of currents in the pnp transistor is just opposite
to the flow of currents in the npn transistor.
In this video, we’ve discussed the basic structure and the basic operation of a bipolar
junction transistor. We learned that a bipolar junction transistor is composed of three doped
semiconductor regions, has two basic types - npn and pnp, and both types have two pn
junctions. We also learned how to bias a bipolar junction transistor in order for it to operate
as an amplifier and discussed what’s happening inside an npn transistor. If you have any
questions, leave it in the comments below and if you’ve found this interesting or
helpful, please subscribe to our channel and like this video!